The time signature is used to show how many beats are in each bar and which note value half-note, quarter-note, eighth-not), and so on will count for one beat.
Time signatures use two numbers, one above the other.
In musical chart (or score), the time signature appears at the beginning of the piece, immediately following the key signature (or immediately following the clef if there is no key signature). Sometimes a time signature may change ex. 3/4 to 4/4 in the middle of a piece of music.
Simple and compound time signatures
A time signature defines the pulse and thus establishes the "count" of a musical work.
Time signatures can be "simple" or "compound".
Simple time signatures
In simple time signatures:
- the upper number indicates how many beats there are in a bar or measure;
- the lower number indicates the note value which represents ONE beat (the "beat unit").
The most common simple time signatures are 2/4, 3/4, and 4/4. The 4 at the bottom indicates that the beat unit is the quarter-note 3/4 means: Three x quarter-note beats per measure.
Compound time signatures
Compound time signatures have an upper number which is commonly 6, 9 or 12. The most common lower number in a compound time signature is 8, representing the eighth note value.
Interpreting compound time signatures
The upper and lower numbers in compound time signatures need to be factored as follows:
- To determine the number of beats per measure, divide the upper number by three. For example, in 6/8, there are 2 beats per measure (because 6 divided by 3 equals 2). So the pulse for the piece is 1-2, 1-2, 1-2, etc.
- To identify the "beat unit" (i.e. which type of note represents ONE beat), multiply the note value represented by the lower number by three. For example, in 6/8, the lower number (8) represents the note value of an eighth-note (or quaver). Multiplying that note value by three gives a beat unit of a dotted quarter-note. (i.e., 3 quavers).
In compound time, the beat unit is almost always a dotted note value. The most common compound time signatures are 6/8 , 9/8 and 12/8, denoting two, three and four x dotted quarter-note beats per measure respectively.
Notwithstanding the above, there are rare occasions when the time signature is interpreted as in simple time, without the factoring. Thus 6/8 would be seen as 6 x quaver beats per bar.
Beat and meter
Time signatures indicating two beats per bar (whether simple or compound) are called duple meter; those with three beats to the bar are triple meter. Terms such as quadruple, septuple, and so on are also occasionally used. See Metre (music).
Most frequent time signatures
| Simple time signatures | |
| 4/4 | common time: widely used in classical music; the norm in rock, jazz, country, and bluegrass, and most modern pop or dance music |
| 2/2 | alla breve, cut time: used for marches and fast orchestral music. Frequently occurs in musical theater. Sometimes called "in 2". |
| 4/2 | rare in music since 1600, although Brahms and other conservative composers used it occasionally. |
| 2/4 | used for polkas or marches |
| 3/4 | used for waltzes, minuets, scherzi, and country & western ballads. |
| Compound time signatures | |
| 6/8 | used for jigs, fast waltzes or marches |
| 9/8 | "compound triple time", occurring rarely (The Sorcerer's Apprentice is a familiar example) |
| 12/8 | common in blues and doo-wop, also used in some jigs. |