Character encoding: Difference between revisions

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{{Short description|Using numbers to represent text characters}}

{{Use dmy dates|date=January 2018}}

[[File:Papertape-Wikipedia-example-dark1-2000px.png|thumb|[[Punched tape]] with the word "Wikipedia" encoded in [[ASCII]]. Presence and absence of a hole represents 1 and 0, respectively; for example, "W" is encoded as "{{code|1010111"}}.]]

'''Character encoding''' is thea processconvention of assigningusing numbersa numeric value to [[Graphics|graphical]]represent each [[character (computing)|characterscharacter]], especially the written characters of a [[LanguageWriting system|humanwriting languagescript]],. allowingNot themonly tocan bea character set include [[Datanatural storage|storedlanguage]], [[DataCharacter communication(symbol)|transmittedsymbols]], but it can also include codes that have meanings or functions outside of language, such as [[control character]]s and [[Computing|transformedwhitespace]]. usingCharacter encodings have also been defined for some [[Digitalconstructed electronics|digitallanguage]]s. When encoded, character data can be stored, transmitted, and transformed by a [[computer]]s.<ref>{{cite web|title = Character Encoding Definition|date = September 24, 2010 |url= http://techterms.com/definition/characterencoding |website=The Tech Terms Dictionary}}</ref> The numerical values that make up a character encoding are known as "[[code point]]s" and collectively comprise a "code space", or a "[[code page]]", or a "[[Character Map (Windows)|character map]]".

Early character codesencodings associatedthat originated with the optical or electrical [[Telegraphy|telegraphtelegraphy]] and in early computers could only represent a subset of the characters used in [[written language]]slanguages, sometimes restricted to [[Letter case|upper case letter]]s, [[Numeral system|numeral]]s and somelimited [[punctuation]] only. TheOver lowtime, costencodings of digital representationcapable of data in modern computer systems allowsrepresenting more elaboratecharacters characterwere codescreated, (such as [[UnicodeASCII]]), which[[ISO/IEC represent8859]], mostand of[[Unicode]] theencodings characterssuch usedas in[[UTF-8]] manyand written languages. Character encoding using internationally accepted standards permits worldwide interchange of text in electronic form[[UTF-16]].

The [[Popularity of text encodings|most usedpopular character encoding]] on the [[World Wide Web|web]] is [[UTF-8]], which is used in 98.2% of surveyed web sites, as of May 2024.<ref name="W3TechsWebEncoding">{{Cite web |title=Usage Survey of Character Encodings broken down by Ranking |url=https://w3techs.com/technologies/cross/character_encoding/ranking |access-date=2024-04-29 |website=W3Techs |language=en}}</ref> In [[Application software|application programs]] and [[operating system]] tasks, both UTF-8 and [[UTF-16]] are popular options.<ref name=":0">{{Cite web |title=Charset |url=https://developer.android.com/reference/java/nio/charset/Charset |access-date=2021-01-02 |website=Android Developers |language=en |quote=Android note: The Android platform default is always UTF-8.}}</ref><ref name=":1">{{Cite web |last=Galloway |first=Matt |date=9 October 2012 |title=Character encoding for iOS developers. Or UTF-8 what now? |url=https://www.galloway.me.uk/2012/10/character-encoding-for-ios-developers-utf8/ |access-date=2021-01-02 |website=www.galloway.me.uk |language=en |quote=in reality, you usually just assume UTF-8 since that is by far the most common encoding.}}</ref>

== History ==

The history of character codes illustrates the evolving need for machine-mediated character-based symbolic information over a distance, using once-novel electrical means. The earliest codes were based upon manual and hand-written encoding and cyphering systems, such as [[Bacon's cipher]], [[Braille]], [[international maritime signal flags]], and the 4-digit encoding of Chinese characters for a [[Chinese telegraph code]] ([[Hans Schjellerup]], 1869). With the adoption of electrical and electro-mechanical techniques these earliest codes were adapted to the new capabilities and limitations of the early machines. The earliest well-known electrically transmitted character code, [[Morse code]], introduced in the 1840s, used a system of four "symbols" (short signal, long signal, short space, long space) to generate codes of variable length. Though some commercial use of Morse code was via machinery, it was often used as a manual code, generated by hand on a [[telegraph key]] and decipherable by ear, and persists in [[amateur radio]] and [[Non-directional beacon|aeronautical]] use. Most codes are of fixed per-character length or variable-length sequences of fixed-length codes (e.g. [[Unicode]]).<ref>{{cite web | url=http://blog.smartbear.com/development/ancient-computer-character-code-tables-and-why-theyre-still-relevant/ | title=Ancient Computer Character Code Tables – and Why They're Still Relevant | publisher=Smartbear | date=17 April 2014 | access-date=29 April 2014 | author=Tom Henderson |url-status=dead |archive-url=https://web.archive.org/web/20140430000312/http://blog.smartbear.com/development/ancient-computer-character-code-tables-and-why-theyre-still-relevant/ |archive-date= Apr 30, 2014 }}</ref>

Common examples of character encoding systems include Morse code, the [[Baudot code]], the [[American Standard Code for Information Interchange]] (ASCII) and Unicode. Unicode, a well-defined and extensible encoding system, has supplantedreplaced most earlier character encodings, but the path of code development to the present is fairly well known.

The Baudot code, a five-[[bit]] encoding, was created by [[Émile Baudot]] in 1870, patented in 1874, modified by Donald Murray in 1901, and standardized by CCITT as International Telegraph Alphabet No.&nbsp;2 (ITA2) in 1930. The name ''baudot'' has been erroneously applied to ITA2 and its many variants. ITA2 suffered from many shortcomings and was often improved by many equipment manufacturers, sometimes creating compatibility issues. In 1959 the U.S. military defined its [[Fieldata]] code, a six-or seven-bit code, introduced by the U.S. Army Signal Corps. While Fieldata addressed many of the then-modern issues (e.g. letter and digit codes arranged for machine collation), it fell short of its goals and was short-lived. In 1963 the first ASCII code was released (X3.4-1963) by the ASCII committee (which contained at least one member of the Fieldata committee, W. F. Leubbert), which addressed most of the shortcomings of Fieldata, using a simpler code. Many of the changes were subtle, such as collatable character sets within certain numeric ranges. ASCII63 was a success, widely adopted by industry, and with the follow-up issue of the 1967 ASCII code (which added lower-case letters and fixed some "control code" issues) ASCII67 was adopted fairly widely. ASCII67's American-centric nature was somewhat addressed in the European [[ECMA-6]] standard.<ref>{{cite web | url=https://www.sr-ix.com/Archive/CharCodeHist/index.html | title=An annotated history of some character codes | date=1 March 2010 | access-date=1 November 2018 | author=Tom Jennings}}</ref>

[[File:Blue-punch-card-front-horiz.png|thumb|Hollerith 80-column punch card with EBCDIC character set]]

[[Herman Hollerith]] invented punch card data encoding in the late 19th century to analyze census data. Initially, each hole position represented a different data element, but later, numeric information was encoded by numbering the lower rows 0 to 9, with a punch in a column representing its row number. Later alphabetic data was encoded by allowing more than one punch per column. Electromechanical [[tabulating machine]]s represented date internally by the timing of pulses relative to the motion of the cards through the machine. When [[IBM]] went to electronic processing, starting with the [[IBM 603]] Electronic Multiplier, it used a variety of binary encoding schemes that were tied to the punch card code.

When [[IBM]] usedwent severalto electronic processing, starting with the [[Binary-coded decimal#IBM|Binary Coded Decimal603]] (Electronic Multiplier, it used a variety of binary encoding schemes that were tied to the punch card code. IBM used several [[BCD (character encoding)|BCDbinary-coded decimal]] (BCD) six-bit character encoding schemes, starting as early as 1953 in its [[IBM 702|702]]<ref>{{cite web|url=http://www.bitsavers.org/pdf/ibm/702/22-6173-1_702prelim_Feb56.pdf |via=bitsavers.org |archive-url=https://ghostarchive.org/archive/20221009/http://www.bitsavers.org/pdf/ibm/702/22-6173-1_702prelim_Feb56.pdf |archive-date=2022-10-09 |url-status=live|title=IBM Electronic Data-Processing Machines Type 702 Preliminary Manual of Information|date=1954|id=22-6173-1|page=80}}</ref> and [[IBM 704|704]] computers, and in its later [[IBM 700/7000 series|7000 Series]] and [[IBM 1400 series|1400 series]], as well as in associated peripherals. Since the punched card code then in use onlywas allowedlimited to digits, upper-case English letters and a few special characters, six bits were sufficient. These BCD encodings extended existing simple four-bit numeric encoding to include alphabetic and special characters, mapping them easily to punch-card encoding which was already in widespread use. IBM's codes were used primarily with IBM equipment;. otherOther computer vendors of the era had their own character codes, often six-bit, butsuch as the encoding used by the {{nobreak|[[UNIVAC I]]}}.<ref>{{cite web|url=http://www.bitsavers.org/pdf/univac/univac1/UnivacI_RefCard.pdf|title=UNIVAC System|type=reference card}}</ref> They usually had the ability to read tapes produced on IBM equipment. TheseIBM's BCD encodings were the precursors of IBM'stheir [[Extended Binary-Coded Decimal Interchange Code]] (usually abbreviated as EBCDIC), an eight-bit encoding scheme developed in 1963 for the [[IBM System/360]] that featured a larger character set, including lower case letters.

In 1959 the U.S. military defined its [[Fieldata]] code, a six-or seven-bit code, introduced by the U.S. Army Signal Corps. While Fieldata addressed many of the then-modern issues (e.g. letter and digit codes arranged for machine collation), it fell short of its goals and was short-lived. In 1963 the first ASCII code was released (X3.4-1963) by the ASCII committee (which contained at least one member of the Fieldata committee, W. F. Leubbert), which addressed most of the shortcomings of Fieldata, using a simpler seven-bit code. Many of the changes were subtle, such as collatable character sets within certain numeric ranges. ASCII63 was a success, widely adopted by industry, and with the follow-up issue of the 1967 ASCII code (which added lower-case letters and fixed some "control code" issues) ASCII67 was adopted fairly widely. ASCII67's American-centric nature was somewhat addressed in the European [[ECMA-6]] standard.<ref>{{cite web | url=https://www.sr-ix.com/Archive/CharCodeHist/index.html | title=An annotated history of some character codes | date= 20 April 2016 |website=Sensitive Research | access-date=1 November 2018 | author=Tom Jennings}}</ref> Eight-bit [[extended ASCII]] encodings, such as various vendor extensions and the [[ISO/IEC 8859]] series, supported all ASCII characters as well as additional non-ASCII characters.

In trying to develop universally interchangeable character encodings, researchers in the 1980s faced the dilemma that, on the one hand, it seemed necessary to add more bits to accommodate additional characters, but on the other hand, for the users of the relatively small character set of the Latin alphabet (who still constituted the majority of computer users), those additional bits were a colossal waste of then-scarce and expensive computing resources (as they would always be zeroed out for such users). In 1985, the average personal computer user's [[hard disk drive]] could store only about 10 megabytes, and it cost approximately US$250 on the wholesale market (and much higher if purchased separately at retail),<ref name="Strelho">{{cite news |last1=Strelho |first1=Kevin |title=IBM Drives Hard Disks to New Standards |url=https://books.google.com/books?id=zC4EAAAAMBAJ&pg=PA29 |access-date=November 10, 2020 |work=InfoWorld |publisher=Popular Computing Inc. |date=April 15, 1985 |pages=29–33}}</ref> so it was very important at the time to make every bit count.

InWhile trying to develop universally interchangeable character encodings, researchers in the 1980s faced the dilemma that, on the one hand, it seemed necessary to add more bits to accommodate additional characters, but on the other hand, for the users of the relatively small character set of the Latin alphabet (who still constituted the majority of computer users), those additional bits were a colossal waste of then-scarce and expensive computing resources (as they would always be zeroed out for such users). In 1985, the average personal computer user's [[hard disk drive]] could store only about 10 megabytes, and it cost approximately US$250 on the wholesale market (and much higher if purchased separately at retail),<ref name="Strelho">{{cite news |last1=Strelho |first1=Kevin |title=IBM Drives Hard Disks to New Standards |url=https://books.google.com/books?id=zC4EAAAAMBAJ&pg=PA29 |access-date=November 10, 2020 |work=InfoWorld |publisher=Popular Computing Inc. |date=April 15, 1985 |pages=29–33}}</ref> so it was very important at the time to make every bit count.

The compromise solution that was eventually found and {{vague|text=developed into Unicode|reason=Became Unicode or was added to Unicode? See talk page.|date=April 2023}} was to break the assumption (dating back to telegraph codes) that each character should always directly correspond to a particular sequence of bits. Instead, characters would first be mapped to a universal intermediate representation in the form of abstract numbers called [[code point]]s. Code points would then be represented in a variety of ways and with various default numbers of bits per character (code units) depending on context. To encode code points higher than the length of the code unit, such as above 256 for eight-bit units, the solution was to implement [[variable-width encoding|variable-length encodings]] where an escape sequence would signal that subsequent bits should be parsed as a higher code point.

== Terminology ==

Informally,The thevarious terms "characterrelated encoding",to "character map", "character set" and "code page"encoding are often used interchangeablyinconsistently or incorrectly.<ref name="SteeleMSDN">{{cite web|url=https://docslearn.microsoft.com/en-us/archive/blogs/shawnste/whats-the-difference-between-an-encoding-code-page-character-set-and-unicode|author=Shawn Steele|title=What's the difference between an Encoding, Code Page, Character Set and Unicode?|date=15 March 2005|website=Microsoft Docs}}</ref> Historically, the same standard would specify a repertoire of characters and how they were to be encoded into a stream of code units — usually with a single character per code unit. However, due to the emergence of more sophisticated character encodings, the distinction between these terms has become important.

===Character===

* A ''[[Character (computing)|character]]'' is a minimal unit of text that has semantic value.<ref name="SteeleMSDN"/><ref name="Unicode glossary">{{cite web |title=Glossary of Unicode Terms |url=https://unicode.org/glossary/ |publisher=Unicode Consortium}}</ref>

* A ''character set'' is athe collectionsmallest unit of elementstext usedthat tohas representsemantic textvalue.<ref name="SteeleMSDN"/><ref name="Unicode glossary"/>{{cite Forweb example,|title=Glossary theof [[LatinUnicode alphabet]]Terms and|url=https://unicode.org/glossary/ [[Greek|publisher=Unicode alphabet]]Consortium}}</ref> are both character sets.

* {{anchor|CCS}}A ''coded character set'' is a character set mapped to set of unique numbers.<ref name="Unicode glossary"/> For historical reasons, this is also often referred to as a [[code page]].<ref name="SteeleMSDN"/>

* {{anchor|repertoire}}A ''character repertoire'' is the set of characters that can be represented by a particular coded character set.<ref name="Unicode glossary"/><ref name="unicode15">{{cite book |title=The Unicode Standard Version 15.0 – Core Specification |date=September 2022 |publisher=Unicode Consortium |isbn=978-1-936213-32-0 |chapter=Chapter 3: Conformance |url=https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf}}</ref> The repertoire may be closed, meaning that no additions are allowed without creating a new standard (as is the case with ASCII and most of the ISO-8859 series); or it may be open, allowing additions (as is the case with Unicode and to a limited extent [[Windows code page]]s).<ref name="unicode15"/>

* A ''code point'' is a value or position of a character in a coded character set.<ref name="Unicode glossary"/>

* A ''code space'' is the range of numerical values spanned by a coded character set.<ref name="Unicode glossary"/><ref name="utr17"/>

* A ''code unit'' is the minimum bit combination that can represent a character in a character encoding (in [[computer science]] terms, it is the [[Word (computer architecture)|word]] size of the character encoding).<ref name="Unicode glossary"/><ref name="utr17"/> For example, common code units include 7-bit, 8-bit, 16-bit, and 32-bit. In some encodings, some characters are encoded using multiple code units; such an encoding is referred to as a [[variable-width encoding]].

What constitutes a character varies between character encodings. For example, for letters with [[diacritic]]s, there are two distinct approaches that can be taken to encode them:. theyThey can be encoded either as a single unified character (known as a precomposed character), or as separate characters that combine into a single [[glyph]]. The former simplifies the text handling system, but the latter allows any letter/diacritic combination to be used in text. [[Typographic ligature|Ligatures]] pose similar problems. Some writing systems, such as Arabic and Hebrew, need to accommodate things like [[grapheme]]s that are joined in different ways in different contexts, but represent the same semantic character.

===Code pages===

{{main|Code page}}

"Code page" is a historical name for a coded character set.

===Character set===

Originally, a code page referred to a specific [[page number]] in the IBM standard character set manual, which would define a particular character encoding.<ref name="DEC_VT510">{{cite web |title=VT510 Video Terminal Programmer Information |at=7.1. Character Sets - Overview |publisher=[[Digital Equipment Corporation]] (DEC) |url=http://www.vt100.net/docs/vt510-rm/chapter7.html#S7.1 |access-date=2017-02-15 |quote=In addition to traditional [[Digital Equipment Corporation|DEC]] and [[ISO]] character sets, which conform to the structure and rules of [[ISO 2022]], the [[VT510]] supports a number of IBM PC code pages ([[page number]]s in IBM's standard character set manual) in [[PCTerm]] mode to emulate the [[console terminal]] of industry-standard PCs. |archive-date=2016-01-26 |archive-url=https://web.archive.org/web/20160126192029/http://www.vt100.net/docs/vt510-rm/chapter7.html#S7.1 |url-status=live }}</ref> Other vendors, including [[Microsoft]], [[SAP AG|SAP]], and [[Oracle Corporation]], also published their own sets of code pages; the most well-known code page suites are "[[Windows code page|Windows]]" (based on Windows-1252) and "IBM"/"DOS" (based on [[code page 437]]).

A character set is a collection of characters used to represent text.<ref name="SteeleMSDN"/><ref name="Unicode glossary"/> For example, the [[Latin alphabet]] and [[Greek alphabet]] are character sets.

==={{anchor|CCS}}Coded character set===

Despite no longer referring to specific page numbers in a standard, many character encodings are still referred to by their code page number; likewise, the term "code page" is often still used to refer to character encodings in general.

A coded character set is a character set with each item uniquely mapped to a numberic value.<ref name="Unicode glossary"/>

Originally,This is also known as a [[code page]],<ref name="SteeleMSDN"/> although that term is generally antiquated. Originally, ''code page'' referred to a specific [[page number]] in thean IBM standard character set manual, which wouldthat definedefined a particular character encoding.<ref name="DEC_VT510">{{cite web |title=VT510 Video Terminal Programmer Information |at=7.1. Character Sets - Overview |publisher=[[Digital Equipment Corporation]] (DEC) |url=http://www.vt100.net/docs/vt510-rm/chapter7.html#S7.1 |access-date=2017-02-15 |quote=In addition to traditional [[Digital Equipment Corporation|DEC]] and [[ISO]] character sets, which conform to the structure and rules of [[ISO 2022]], the [[VT510]] supports a number of IBM PC code pages ([[page number]]s in IBM's standard character set manual) in [[PCTerm]] mode to emulate the [[console terminal]] of industry-standard PCs. |archive-date=2016-01-26 |archive-url=https://web.archive.org/web/20160126192029/http://www.vt100.net/docs/vt510-rm/chapter7.html#S7.1 |url-status=live }}</ref> Other vendors, including [[Microsoft]], [[SAP AG|SAP]], and [[Oracle Corporation]], also published their own sets of code pages;, theincluding mostnotable well-known[[Windows code page]] suitesand are "[[Windows code page|Windows 437]]". (basedDespite onno Windows-1252)longer andreferring "IBM"/"DOS"to (basedspecific onpages [[in a manual, many character encodings are still identified to by the same number. Likewise, the term ''code page'' 437]])is still used to refer to character encoding.

The term "code page" is not used in Unix or Linux, where "charmap" is preferred, usually in the larger context of locales. IBM's Character Data Representation Architecture (CDRA) designates entities with coded character set identifiers ([[CCSID]]s), each of which is variously called a "charset", "character set", "code page", or "CHARMAP".<ref name=utr17/>

In [[Unix]] and [[Unix-like]] systems, the term ''charmap'' is commonly used; usually in the larger context of locales.

===Code units===

The code unit size is equivalent to the bit measurement for the particular encoding:

* A code unit in [[ASCII]] consists of 7 bits;

* A code unit in [[UTF-8]], [[EBCDIC]] and [[GB 18030]] consists of 8 bits;

* A code unit in [[UTF-16]] consists of 16 bits;

* A code unit in [[UTF-32]] consists of 32 bits.

The term "code page" is not used in Unix or Linux, where "charmap" is preferred, usually in the larger context of locales. IBM's Character Data Representation Architecture (CDRA) designates entitieseach entity with a [[CCSID|coded character set identifiersidentifier ([[CCSID)]]s), each of which is variously called a "''charset"'', "''character set"'', "''code page"'', or "''CHARMAP"''.<ref name=utr17/>

===Code points===

A code point is represented by a sequence of code units. The mapping is defined by the encoding. Thus, the number of code units required to represent a code point depends on the encoding:

==={{anchor|repertoire}}Character repertoire===

* {{anchor|repertoire}}A ''character repertoire'' is thea set of characters that can be represented by a particular coded character set.<ref name="Unicode glossary"/><ref name="unicode15">{{cite book |title=The Unicode Standard Version 15.0 – Core Specification |date=September 2022 |publisher=Unicode Consortium |isbn=978-1-936213-32-0 |chapter=Chapter 3: Conformance |url=https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf}}</ref> The repertoire may be closed, meaning that no additions are allowed without creating a new standard (as is the case with ASCII and most of the ISO-8859 series); or it may be open, allowing additions (as is the case with Unicode and to a limited extent [[Windows code page]]s).<ref name="unicode15"/>

===Code pagespoint===

{{main|Code pagepoint}}

A [[code point]] is the value or position of a character in a coded character set.<ref name="Unicode glossary"/> A code point is represented by a sequence of code units. The mapping is defined by the encoding. Thus, the number of code units required to represent a code point depends on the encoding:

* UTF-8: code points map to a sequence of one, two, three or four code units.

* UTF-16: code units are twice as long as 8-bit code units. Therefore, any code point with a scalar value less than U+10000 is encoded with a single code unit. Code points with a value U+10000 or higher require two code units each. These pairs of code units have a unique term in UTF-16: [[UTF-16#Code points from U+010000 to U+10FFFF|"Unicode surrogate pairs".]]

Line 60 ⟶ 58:

* GB 18030: multiple code units per code point are common, because of the small code units. Code points are mapped to one, two, or four code units.<ref>{{cite web | url=https://docs.oracle.com/javase/tutorial/i18n/text/terminology.html | title=Terminology (The Java Tutorials) | publisher=Oracle | access-date=25 March 2018 }}</ref>

===CharactersCode space===

*Code A ''code point''space is athe value or positionrange of anumerical charactervalues spanned inby a coded character set.<ref name="Unicode glossary"/><ref name="utr17"/>

{{main|Character (computing)}}

Exactly what constitutes a character varies between character encodings.

===Code unitsunit===

For example, for letters with [[diacritic]]s, there are two distinct approaches that can be taken to encode them: they can be encoded either as a single unified character (known as a precomposed character), or as separate characters that combine into a single [[glyph]]. The former simplifies the text handling system, but the latter allows any letter/diacritic combination to be used in text. [[Typographic ligature|Ligatures]] pose similar problems.

* A ''code unit'' is the minimum bit combination that can represent a character in a character encoding (in [[computer science]] terms, it is the [[Word (computer architecture)|word]] size of the character encoding).<ref name="Unicode glossary"/><ref name="utr17"/> For example, commonCommon code units include 7-bit, 8-bit, 16-bit, and 32-bit. In some encodings, some characters are encoded using multiple code units; such an encoding is referred to as a [[variable-width encoding|multiple code units]].

For example:

Exactly how to handle [[glyph]] variants is a choice that must be made when constructing a particular character encoding. Some writing systems, such as Arabic and Hebrew, need to accommodate things like [[grapheme]]s that are joined in different ways in different contexts, but represent the same semantic character.

* [[ASCII]]: 7 bits

* A code unit in [[UTF-8]], [[EBCDIC]] and [[GB 18030]] consists of: 8 bits;

* A code unit in [[UTF-16]] consists of: 16 bits;

* A code unit in [[UTF-32]] consists of: 32 bits.

==Unicode encoding model==

[[Unicode]] and its parallel standard, the ISO/IEC 10646 [[Universal Character Set]], together constitute a unified standard for character encoding. Rather than mapping characters directly to [[byte]]s, Unicode separately defines a coded character set that maps characters to unique natural numbers ([[code point]]s), how those code points are mapped to a series of fixed-size natural numbers (code units), and finally how those units are encoded as a stream of octets (bytes). The purpose of this decomposition is to establish a universal set of characters that can be encoded in a variety of ways. To describe thisthe model precisely, Unicode uses itsexisting ownterms setand ofdefines terminologynew to describe its process:terms.<ref name="utr17">{{cite web |last1=Whistler |first1=Ken |last2=Freytag |first2=Asmus |title=UTR#17: Unicode Character Encoding Model |url=https://www.unicode.org/reports/tr17/ |publisher=Unicode Consortium |access-date=12 August 2023 |date=2022-11-11}}</ref>

===Abstract character repertoire===

An '''abstract character repertoire''' (ACR) is the full set of abstract characters that a system supports. Unicode has an open repertoire, meaning that new characters will be added to the repertoire over time.

===Coded character set===

A '''coded character set''' (CCS) is a [[function (mathematics)|function]] that maps characters to ''[[code point]]s'' (each code point represents one character). For example, in a given repertoire, the capital letter "A" in the Latin alphabet might be represented by the code point 65, the character "B" by 66, and so on. Multiple coded character sets may share the same character repertoire; for example [[ISO/IEC 8859-1]] and IBM code pages 037 and [[Code page 500|500]] all cover the same repertoire but map them to different code points.

===Character encoding form===

A '''character encoding form''' (CEF) is the mapping of code points to ''code units'' to facilitate storage in a system that represents numbers as bit sequences of fixed length (i.e. practically any computer system). For example, a system that stores numeric information in 16-bit units can only directly represent code points 0 to 65,535 in each unit, but larger code points (say, 65,536 to 1.4&nbsp;million) could be represented by using multiple 16-bit units. This correspondence is defined by a CEF.

===Character encoding scheme===

A '''character encoding scheme''' (CES) is the mapping of code units to a sequence of octets to facilitate storage on an octet-based file system or transmission over an octet-based network. Simple character encoding schemes include [[UTF-8]], [[UTF-16BE]], [[UTF-32BE]], [[UTF-16LE]], and [[UTF-32LE]]; compound character encoding schemes, such as [[UTF-16]], [[UTF-32]] and [[ISO/IEC 2022]], switch between several simple schemes by using a [[byte order mark]] or [[escape sequence]]s; compressing schemes try to minimize the number of bytes used per code unit (such as [[Standard Compression Scheme for Unicode|SCSU]] and [[Binary Ordered Compression for Unicode|BOCU]]).

Although [[UTF-32BE]] and [[UTF-32LE]] are simpler CESes, most systems working with Unicode use either [[UTF-8]], which is [[backward compatibility|backward compatible]] with fixed-length ASCII and maps Unicode code points to variable-length sequences of octets, or [[UTF-16BE]],{{cn|date=August 2023}} which is [[backward compatibility|backward compatible]] with fixed-length UCS-2BE and maps Unicode code points to variable-length sequences of 16-bit words. See [[comparison of Unicode encodings]] for a detailed discussion.

===Higher-level protocol===

Finally, thereThere may be a '''higher-level protocol''' which supplies additional information to select the particular variant of a [[Unicode]] character, particularly where there are regional variants that have been 'unified' in Unicode as the same character. An example is the [[XML]] attribute xml:lang.

The Unicode model uses the term "character map" for other systems which directly assign a sequence of characters to a sequence of bytes, covering all of the CCS, CEF and CES layers.<ref name="utr17" />

===UnicodeCode codepoint pointsdocumentation===

In Unicode, aA character canis becommonly referred todocumented as 'U+' followed by its codepointcode point value in [[hexadecimal]]. The range of valid code points (the codespacecode space) for the Unicode standard is U+0000 to U+10FFFF, inclusive, divided in 17 [[Plane (Unicode)|planes]], identified by the numbers 0 to 16. Characters in the range U+0000 to U+FFFF are in plane 0, called the [[Plane (Unicode)#Basic Multilingual Plane|Basic Multilingual Plane]] (BMP). This plane contains the most commonly- used characters. Characters in the range U+10000 to U+10FFFF in the other planes are called [[supplementary characters]].

The following table showsincludes examples of code point valuespoints:

{| class="wikitable MsoNormalTable"

! Character

! Unicode codeCode point

! Glyph

|-

Line 120 ⟶ 127:

===Example===

Consider a [[String (computer science)|string]] of the letters, "ab̲c𐐀"—that is,{{endash}} a string containing a Unicode combining character ({{unichar|0332}}) as well as a supplementary character ({{unichar|10400}}). This string has several Unicode representations which are logically equivalent, yet while each is suited to a diverse set of circumstances or range of requirements:

* Four [[Character (computing)|composed characters]]:

*:{{code|a}}, {{code|b̲}}, {{code|c}}, {{code|𐐀}}

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== Transcoding ==

AsTo a resultsupport ofenvironments havingusing manymultiple character encoding methods in use (and the need for backward compatibility with archived data)encodings, many computer programssoftware havehas been developed to translate datatext between character encoding schemes,; a process known as [[transcoding]]. SomeNotable ofsoftware these are cited below.includes:

* [[Web browser]]s – most modern webModern browsers feature automatic [[character encoding detection]]. On Firefox 3, for example, see the View/Character Encoding submenu.

[[Cross-platform]]:

* [[iconv]] – a programProgram and standardized API to convert encodings

* [[Web browser]]s – most modern web browsers feature automatic [[character encoding detection]]. On Firefox 3, for example, see the View/Character Encoding submenu.

* [[luit]] – a programProgram that converts encoding of input and output to programs running interactively

* [[iconv]] – a program and standardized API to convert encodings

* [[International Components for Unicode]] – A set of C and Java libraries to performfor charset conversion. uconv can be used from ICU4C.

* [[luit]] – a program that converts encoding of input and output to programs running interactively

* Encoding.Convert – [[.NET]] API<ref>{{cite web|url=https://docs.microsoft.com/en-us/dotnet/api/system.text.encoding.convert?redirectedfrom=MSDN&view=net-6.0#overloads|work=Microsoft .NET Framework Class Library|title=Encoding.Convert Method}}</ref>

* [[International Components for Unicode]] – A set of C and Java libraries to perform charset conversion. uconv can be used from ICU4C.

* MultiByteToWideChar/WideCharToMultiByte – to[[Windows convertAPI]] fromfunctions ANSIfor toconverting Unicodebetween &ANSI and Unicode to ANSI<ref>{{cite web|url=https://learn.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-multibytetowidechar|title=MultiByteToWideChar function (stringapiset.h)|website=Microsoft Docs|date=13 October 2021 }}</ref><ref>{{cite web|url=https://learn.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-widechartomultibyte|title=WideCharToMultiByte function (stringapiset.h)|website=Microsoft Docs|date=9 August 2022 }}</ref>

[[Microsoft Windows|Windows]]:

* Encoding.Convert – .NET API<ref>{{cite web|url=https://docs.microsoft.com/en-us/dotnet/api/system.text.encoding.convert?redirectedfrom=MSDN&view=net-6.0#overloads|work=Microsoft .NET Framework Class Library|title=Encoding.Convert Method}}</ref>

* MultiByteToWideChar/WideCharToMultiByte – to convert from ANSI to Unicode & Unicode to ANSI<ref>{{cite web|url=https://learn.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-multibytetowidechar|title=MultiByteToWideChar function (stringapiset.h)|website=Microsoft Docs|date=13 October 2021 }}</ref><ref>{{cite web|url=https://learn.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-widechartomultibyte|title=WideCharToMultiByte function (stringapiset.h)|website=Microsoft Docs|date=9 August 2022 }}</ref>

== Common character encodings ==

{{Main|Popularity of text encodings}}

{{Expand section|Popularity and comparison:

* Statistics on popularity

* Especially, a comparison of the advantages and disadvantages of the few 3-5 most common character encodings (e.g. UTF-8, UTF-16 and UTF-32)|date=June 2024}}

The [[Popularity of text encodings|most used character encoding]] on the [[World Wide Web|web]] is [[UTF-8]], used in 98.2% of surveyed web sites, as of May 2024.<ref name="W3TechsWebEncoding" /> In [[Application software|application programs]] and [[operating system]] tasks, both UTF-8 and [[UTF-16]] are popular options.<ref name=":0" /><ref name=":1">{{Cite web |last=Galloway |first=Matt |date=9 October 2012 |title=Character encoding for iOS developers. Or UTF-8 what now? |url=https://www.galloway.me.uk/2012/10/character-encoding-for-ios-developers-utf8/ |access-date=2021-01-02 |website=Matt Galloway |language=en |quote=in reality, you usually just assume UTF-8 since that is by far the most common encoding. }}</ref>

{{Div col|colwidth=30em}}

* [[ISO/IEC 646|ISO 646]]

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** [[ISO/IEC 8859-15|ISO 8859-15]] Added the Euro sign and other rationalisations to ISO 8859-1

** [[ISO/IEC 8859-16|ISO 8859-16]] Central, Eastern and Southern European languages (Albanian, Bosnian, Croatian, Hungarian, Polish, Romanian, Serbian and Slovenian, but also French, German, Italian and Irish Gaelic)

* [[Code page 437|CP437]], [[Code page 720|CP720]], [[Code page 737|CP737]], [[Code page 850|CP850]], [[Code page 852|CP852]], [[Code page 855|CP855]], [[Code page 857|CP857]], [[Code page 858|CP858]], [[Code page 860|CP860]], [[Code page 861|CP861]], [[Code page 862|CP862]], [[Code page 863|CP863]], [[Code page 865|CP865]], [[Code page 866|CP866]], [[Code page 869|CP869]], [[Code page 872|CP872]]

* [[Windows code page|MS-Windows character sets]]:

** [[Windows-1250]] for Central European languages that use Latin script, (Polish, Czech, Slovak, Hungarian, Slovene, Serbian, Croatian, Bosnian, Romanian and Albanian)

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== See also ==

* [[{{Annotated link|Percent-encoding]]}}

* [[{{Annotated link|Alt code]]}}

* [[{{Annotated link|Character encodings in HTML]]}}

* {{Annotated link|Charset sniffing}}

* [[:Category:Character encoding]] – articles related to character encoding in general

* [[:Category:Character sets]] – articles detailing specific character encodings

* [[Hexadecimal#Representing{{Annotated hexadecimallink|Hexadecimal representations]]}}

* {{Annotated link|Mojibake}}

* ''[[Mojibake]]'' – character set mismap

* {{Annotated link|Mojikyō}}

* [[Mojikyō]] – a system ("glyph set") that includes over 100,000 Chinese character drawings, modern and ancient, popular and obscure

* [[{{Annotated link|Presentation layer]]}}

* {{Annotated link|TRON (encoding)}}

* [[TRON (encoding)|TRON]], part of the TRON project, is an encoding system that does not use Han Unification; instead, it uses "control codes" to switch between 16-bit "planes" of characters.

* [[{{Annotated link|Universal Character Set characters]]}}

* [[Charset sniffing]] – used in some applications when character encoding metadata is not available

== References ==

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{{DEFAULTSORT:Character Encoding}}

[[Category:Character encoding| ]]

[[Category:Digital typography]]

[[Category:Natural language and computing|Encoding]]