A time signature is a sign written at the beginning of a song (after the clef and key signature) which indicates the time and meter to be played.
They are also written anywhere a new time or meter is to occur.
Figure 1: A basic time signature.
Time signatures contains two numbers; one above the other. The bottom being the denominator and the top the numerator.
The denominator (bottom number) indicates which note represents one unit of measurement.
The following is a list of denominators with their corresponding notes:
The numerator (top number) simply indicates how many of the selected denominator per bar.
To master these two factors is to master the basics of time signatures:
Figure 2: Three different time signatures with their bars complete.
Time signatures are divided into three categories: simple time, compound time, and irregular time.
Simple time signatures are those where each beat has two equal subdivisions. They may be duple, triple, or quadruple:
Compound time signatures are those where each beat has three equal subdivisions. They may be duple, triple, or quadruple:
Irregular (or complex) time signatures are those where each beat is not subdivided by two or three, but rather a mixture of the two. They have meters which do not indicate duple, triple, or quadruple, but rather quintuple (five beats per bar), septuple (seven beats per bar), etc:
Also see Note & Rest Values | Tempo.