Meter is represented with a time signature, whose form is one number on top of another, such as 4/4, which is common time. Common time is so called because many pieces of music have been written using it. The top number of a time signature indicates the number of beats in a bar. The bottom number indicates the type of rhythm value. If 2 is the bottom number, the rhythm value is the half note. Four indicates a quarter note. Eight means eighth note and 16 means sixteenth note.
The most often used simple time signatures are 2/4, 3/4, and 4/4. 2/4 means two quarter notes in a bar. 3/4 means three quarter notes in a bar. 4/4 means four quarter notes in a bar. Less often used simple time signatures are 2/8, 3/8 and 4/8. These mean respectively two, three and four eighth notes in a bar. 2/2, 3/2 and 4/2 are also simple time signatures used less often. These mean respectively two, three and four half notes in a bar.
Compound time signatures are those where the top number is 6, 9, 12 or, less frequently, any other subsequent multiple of three, according to musician William Duckworth. 6/4, 9/4 and 12/4 mean, respectively, six, nine and 12 quarter notes in a bar. 6/8, 9/8 and 12/8 mean, respectively, six, nine and 12 eighth notes in a bar. 6/16, 9/16 and 12/16 mean, respectively, six, nine and 12 sixteenth notes in a bar.
When there are groups of three beats together, as in compound time, it often leads to a compound rhythm value, or dotted note. A dot after a note has the effect of making the note last half as long again. For example, in 6/8 time, the 6 indicates that there are two dotted pulses. The 8 indicates that the pulse is a dotted quarter note.