Time signatures are usually made up of two numbers, one above the other. The top number of a time signature designates the number of beats in a measure. The bottom number designates the reciprocal of the portion of a whole note which represents one beat. For example, a time signature of 3/8 designates three beats per measure, one eighth note to a beat (1/8 of a whole note). A 4 on the bottom indicates that a beat is represented as a quarter note (or quaver), a 2 indicates a half note (or minim), an 16 indicates an sixteenth note (or semiquaver) and so on.
In writing about time signatures outside from notating music, time signatures are generally written with the top number separated from the bottom by a slash (in the manner of a fraction). The example here, for example, can be written 3/4. However in writing the music itself, the time signature is never written as a fraction. When a time signature is represented as a fraction, that fraction represents the portion of a whole note that occupies one measure. If the top and bottom numbers are equal, one whole note occupies one measure.
As a rule, the time signature is written at the beginning of a piece immediately following the key signature on every staff (or immediately following the clef if there is no key signature). If the piece changes metre, this may be indicated by a new time signature in the music.
Time signatures can be "simple" or "compound". The most common simple time signatures are 2/4, 3/4 and 4/4. Here, the top number indicates how many beatss there are in a bar, and the bottom number indicates the length of that beat.
Compound time signatures are distinguished by a top number which is 6 or above and a multiple of three (most commonly 6, 9 or 12). With a compound time signature, the notes are grouped into lengths equal to three beats. A compound time signature is obtained from a simple time signature by multiplying the top number by three and the bottom number by two. These may be counted in groups of three instead of individual beats.
For example, multiplying the simple 2/4 (two quarter notes per bar) by 3/2 obtains a compound time signature of 6/8; this indicates there are six eighth notes in each bar, but they are in two groups of three, each a dotted-quarter note (three eighth notes) in length. Similarly, 9/16 (nine sixteenth notes grouped into three dotted-eighth notes) is the comound equivalent of 3/8 (three eighth notes).
In all cases, the first beat of a bar is stressed, and in time signatures with four groups in the bar (such as 4/4 and 12/8), the third beat is also stressed, though to a lesser degree. This gives a regular pattern of stressed and unstressed beats. If the pattern of stresses is shifted so that a stressed falls on a normally unstressed beat, the rhythm is said to be syncopated.
To indicate more complex patterns of stresses, such as additive rhythms, more complex time signatures can be used. For example, the signature
In some cases, the letter C (common time) is used in place of the 4/4 time signature. A similar C with a vertical line through it can be used in place of 2/2.
Pieces with two beats to the bar, such as 2/4 or 6/8, are said to be in duple meter. Similarly, music with three beats to the bar (such as 3/2 or 9/8) is in triple meter. Music with four beats to the bar is in quadruple meter, five beats is quintuple meter and seven is septuple meter. These names can be combined with the simple and compound terms, so that 3/4 time can be described as simple triple, 6/8 as compound duple and so on.
There is a sense in which all simple triple time signatures, be they 3/8, 3/4, 3/2 or anything else, and all compound duple times, such as 6/8, 6/16 and so on, are equivalent - a piece in 3/4 can be easily rewritten in 3/8 simply by halving the length of the notes. Sometimes, the choice of base note is simply down to tradition: the minuet, for example, is generally written in 3/4, and though examples in 3/8 do exist, a minuet in 3/2 would be highly unconventional. At other times, the choice of bottom note can give subtle hints as to the character of the music: for example, time signatures with a longer base-note (such as 3/2) can be used for pieces in a quick tempo to convey a sense of the time flying by.
Similarly, a piece in 2/4 can often sound like it is in 4/4 (or vice versa) and a piece in simple 3/4 can sound like it is in compound 6/8 or 12/8 time, particularly if the former is played quickly or the latter slowly.
In modern Western Music, in styles such as serialism and minimalism, the time signature is often avoided entirely (the key signature is also frequently omitted). An underlying time signature or key may be present, but it may be too notationally complex or too redundant to notate these details. In other cultures, time is maintained not by a defined notation, but by a drum or other percussion instrument. Examples of this can be found in Indian classical music (see Indian music) and gamelan music, both of which often rely on oral tradition to pass down popular songs, rather than notation, as in Western classical music (see Western music).
Some standard time signatures in Western music are
1 The theme songs from the M:I feature films (1996 and 2000) use 4/4 by repeating the first three beats of the bass line twice, holding melody notes during that period, and halving each note's duration.