The meter of music is its underlying rhythmic shape, or pulse. Sometimes, it stays the same throughout a piece of music. This is often the case with pop songs. Other kinds of song sometimes use the same meter throughout too, for example many of Robert Schumann's lieder. Longer musical works, such as jazz improvisations or classical symphonies, often use a mixtures of meters, which may include simple and compound time.
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.
Simple Time Signatures
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
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.