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It is called Compound Meter when beats in a musical piece are subdivided in three smaller beats.
The compound time signature is easy to recognize, since it will have the number 6, 9 or 12 on top, (numbers that can be divided by 3).
Let’s take a closer look!
Musical meter is how the beats are organized in a piece of music. But the beats can be organized on different levels.
First of all, the main beats of a piece form “groups” depending on where the stronger beats fall.
The most common groupings are in 2's, 3's and 4's, called duple, triple or quadruple meter.
2 beats per group: "loud-soft-loud-soft" (Like a March). This is called Duple meter:
3 beats per group: "loud-soft-soft" (Like a waltz). This is called Triple meter:
4 beats per group: "loud-soft-soft-soft" (Most common). This is called Quadruple meter:
Next, each main beat can also be felt as subdivided in either 2 smaller beats or 3 smaller beats. If subdivided in 2 smaller beats, it is called Simple Meter:
With the sub division in 3 smaller beats it is called Compound Meter:
When meter is written in the score, it is called a Time Signature.
The Compound time signatures are, as mentioned above, easy to spot. They have the numbers 6, 9 or 12 on top. Some commonly used are:
The top number tells us the number of beats per measure. The bottom number tells us what note value is worth 1 beat.
It can be any note value, really, but the most common here are 4 (=quarter note), 8 (=eight note), or 16 (=sixteenth note).
What's interesting is that the beats are grouped in 3's.
So in time signatures with 6 on top, the beats are grouped 3 + 3. Since it is two groups, this is Duple meter, or in this case Compound Duple.
Time signatures with 9 on top are in triple time (3+3+3):
And time signatures with 12 on top are in quadruple time.(3+3+3+3)
Compared to Simple Time Signatures, (where we have to imagine the subdivision of the beat in 2’s to be sure it is not compound), Compound Time Signatures show all the "smaller" beats!
We have to group the beats in 3’s to understand if it is in Duple, Triple or Quadruple time. The main beats in compound time could also be seen as dotted note values: