The meter signature or time signature is an indicator of the meter of a musical work, usually presented in the form of a fraction, the denominator of which indicates the unit of measurement and the numerator of which indicates the number of units that make up a measure. The time signature is typically located at the beginning of the first musical staff of a piece or movement, just after the key signature.
The denominator of the time signature indicates which note value represents the basic beat, or tactus, what a conductor typically would beat. In this position, one most often finds the numbers 2, 4, 8, or 16. These numbers represent, respectively, the half, quarter, eighth, and sixteenth notes. On occasion it contains a 1 (representing a whole note) or 32 (thirty-second note), and very rarely 64 (sixty-fourth note). While numbers falling outside of this two-square pattern (i.e. 3 or 5 or 17) can theoretically make up the denominator, musicians are not trained to understand this notation, as it goes against the grain of Western rhythmic principles, and thus it almost is never written this way by composers.
The numerator of the time signature indicates how many of a given tactus make up a measure. Thus, "common-time," or means four quarter notes make up one measure. This number is subject to much more variety than the denominator, but with numbers between 2 and 8 being most common. Increasingly since the late 19th-century, composers have made frequent use of the so-called "odd meters" such as or .
Time signatures do more than represent the basic facts of the meter, they also give clues to the musician as to style and character. Since composers have a variety of choices within this system, in that a triple meter could be written in , , or , the composer makes a conscious choice, based on whether the piece has a broad or lively character. For example, time, often referred to as "cut-time," is the meter of choice for marches, so a musical work written in this movement might suggest a march-like interpretation even if the word "march" were not in the title. Some intelligent deduction is required, however, because it is also the meter of pavanes, a slow-moving dance of which Wagner's wedding march (written in !) is a well-known example.
Two more trends in 20th century music bear mention. The first is the use of frequently changing meter, first used extensively by Stravinsky, who in his Concerto for Piano and Winds wrote passages where every measure is in a different meter. With this practice, it is debatable if a discernible meter is ever established, and composers often seem to use this tool to evade any sense of meter. On quite the opposite side, another trend is for the use of "additive" time signatures, which appear like this:
What this means is that each measure contains beats of different lengths. Often with this type of time signature, the bottom number does not represent the tactus, rather the smallest rhythmic unit, which is grouped in various configurations into larger pulses. This music often has a discernible groove, and many other musical traditions in Eastern Europe and South America are based on this kind of rhythmic structure.