One of the more confusing aspects of learning to read music for many music students is compound meter. Also called compound time, I've seen virtually all music students and even many music teachers fail to understand its basics. This lesson will clear up the confusion once and for all.
Any time signature in which the top number is a multiple of 3 (but not equal to 3) is an example of compound meter.
Thus, if the top number of a time signature is 6, 9 or 12 (other numbers are almost never encountered), that time signature is a compound meter.
The most important thing to learn about time signatures in general is that the top number does not indicate the number of beats per measure!
Only in the case of simple meter does the top number equal the number of beats in each measure.
To understand why this is the case, let's take a look at some examples of compound meter. The most common one is 6/8:
It's tempting to assume that there are six beats per measure in 6/8 meter, but this isn't the case. To understand this point, let's look at a familiar example:
Sing "Row, row, row your boat" and tap the beats. Notice how you don't tap each syllable of "Merrily, merrily, merrily" but only the first syllable? This is because the song is in two beats.
Each beat happens to total a dotted quarter note. A dotted quarter note has the value of a quarter note plus an eighth note. Since two eighth notes have the rhythmic value of a quarter note, a dotted quarter note also has the same rhythmic value as three eighth notes.
Thus, in 6/8 time, there are a total of six eighth notes per measure, but only two beats, each having the duration of a dotted quarter note.
This is the essence of compound meter. The beat unit is not represented by the lower number of the time signature. Thus, for 6/8 meter, an eighth note is not the beat. Similarly, for 9/4 meter, a quarter note is not the beat.
Let's use 9/16 as an example. Can you figure out the beat unit?
Since the top number, 9, is a multiple of three, this is an example of compound meter. Nine divided by three equals three beats per measure.
Each beat totals three sixteenth notes, since 16 is the bottom number of the time signature. Three sixteenth notes total a dotted eighth note.
Thus, in 9/16 meter, there are three dotted eighth note beats:
Compound meter is covered extensively in my DVD and workbook course, How to Read Sheet Music!