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A '''prefix code''' is a type of [[code]] system (typically a [[variable-length code]]) distinguished by its possession of the "prefix property"; which states that there is no valid [[code word]] in the system that is a [[prefix (computer science)|prefix]] (start) of any other valid code word in the set. For example, a code with code words {9, 59, 55} has the prefix property; a code consisting of {9, 5, 59, 55} does not, because "5" is a prefix of both "59" and "55". With a prefix code, a receiver can identify each word without requiring a special marker between words.
Prefix codes are also known as '''prefix-free codes''', '''prefix condition codes''' and '''instantaneous codes'''. Although [[Huffman coding]] is just one of many algorithms for deriving prefix codes, prefix codes are also widely referred to as "Huffman codes", even when the code was not produced by a Huffman algorithm. The term '''comma-free code''' is sometimes also applied as a synonym for prefix-free codes<ref>US [[Federal Standard 1037C]]</ref><ref>{{citation|title=ATIS Telecom Glossary 2007|url=http://www.atis.org/glossary/definition.aspx?id=6416|accessdate=December 4, 2010}}</ref> but in most mathematical books and articles (e. g.<ref>{{citation|last1=Berstel|first1=Jean|last2=Perrin|first2=Dominique|title=Theory of Codes|publisher=Academic Press|year=1985}}</ref><ref>{{citation|doi=10.4153/CJM-1958-023-9|last1=Golomb|first1=S. W.|author1-link=Solomon W. Golomb|last2=Gordon|first2=Basil|author2-link=Basil Gordon|last3=Welch|first3=L. R.|title=Comma-Free Codes|journal=Canadian Journal of Mathematics|volume=10|issue=2|pages=202–209|year=1958|url=http://books.google.com/books?id=oRgtS14oa-sC&pg=PA202}}</ref>) it is used to mean [[self-synchronizing code]]s, a subclass of prefix codes.
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