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Hello I am learning musical theory, I have a question about compound time signatures :

I read that for a compound time signature, like 6/8,

"the upper number is the number of divisions thus you have to divide it by three to get the number of beats".

This is true if for example each beat is made of 3 divisions, what if each beats is say made of 5 divisions, like a dotted full do + full re beamed together ? In this case you have to divide it by 5, and if each beat is of variable number of divisions, the upper number can't get you immediately the number of beats.

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Let's consider a complex time signature like 7/8. There are 3 different beat divisions we could use: 2,2,3; 2,3,2 and 3,2,2. In practice there are two ways to distinguish between these different possibilities - either study the grouping of notes or see if the composer left a note on the score. The latter's very straight forward. Often modern composers will write above the stave "(2+2+3)", for example. The former requires looking at barred notes and ties and seeing where they split.

In general, the time signature itself only really tells us the number (6) and the length (8) of beats. 6/8 is assumed to mean 3+3 because we've decided it does.

  • I wrote this on mobile. I'll add some score extracts when I have a chance. – Natalie S Sep 22 '14 at 11:00
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    I mean the information isn't directly in the signature. It has been defined that way. And yeah that was a little slip. – Natalie S Sep 22 '14 at 16:02
  • Note also that the beat divisions can vary from bar to bar; for example, the finale of Stravinsky's "Firebird" changes back & forth between 3+2+2 and 2+2+3. – Michael Seifert Dec 5 '16 at 20:04
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Compound time is essentially made up of dotted beats. so the compound version of say 2/4 is 6/8, 1/4 is 3/8, 3/4 is 9/8 and so on... (for 2/2 it's 6/4) meaning there are n dotted beats in a bar (where n=1,2,3 and the beats are as specified by the bottom number) in other words the compound time signature will always be divisible by 3 as a dotted note contains three subnotes (i.e. a dotted crotchet will have three quavers) So 5/8 is not a compound time as far as I'm concerned. I am also an amateur, so I hope a more experienced musician will correct me if there's any mistake.

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6/8 is compound, in that it can be counted in two different ways. It contains 6 quavers (1/8 notes) - just as could be found in 3/4 time, BUT the emphasis is not the same. In 3/4, the count is 1and 2and 3and, but in 6/8 it's ONE,two, three,FOUR, five, six. Could be exactly the same tempo, but on 6/8 the 'feel' is different as in there are two emphasised parts - ONE and FOUR. This effectively gives it a 'two' feel, as it can be counted as ONE, TWO, slower than the quick six count. Thus your idea of divide by 3. Exactly the same thing happens with 3/8, 9/8 and 12/8. 12/8 will be countable as ONE-- TWO--THREE--FOUR--.So, yes, each beat has 3 divisions, in a way.

5/8, 5/4, 7/8 and 7/4 are not compound, as they cant be 'split' in the same way. Yes, 5/4 is often split into ONE, two, three, FOUR,five, but the parts aren't equal. It gets split into ONE, two, THREE, four, five as well, which gives a different feel.Each beat has not five divisions, each BAR has five !

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We have notes available that split readily into two - quarter notes, half notes... We have notes that split readily into three - dotted quarters, dotted halves... There isn't a stock five-divisible note. If that's what you want, use tuplet notation.

In traditional theory there are two types of time - Simple and Compound. Simple has beats that divide into 2, Compound has beats that divide into three.

Why don't we have a type that divides into 5? Well, we could. Notational possibilities are wide open these days, and some composers delight in unusual time signatures like 7/13 and 9/6. (They usually use them to indicate tempo relationships between different sections of music, rather than any particularly complex rhythm within the bar though. Don't worry about it for now!)

But you'll find that music tends to fall into groups of 2 or 3. 5/8 is not a pure Simple or Compound time, it's either 3+2, a compound beat followed by a simple one, or 2+3, a simple beat followed by a compound one.

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You're right. It would be possible to divide the beat into 5. One example is C-V Alkan's Etude in E op 35 no 12. This is notated in 10/16. Its tempo is given as "Andante. MM 88=crotchet [quarter] tied to semiquaver [16th]". But compound time-signatures with each beat divided into 5 or any higher prime are IME extremely rare, so mentioning only the case of division into 3 makes a reasonable simplification which covers all the music that the learner is likely to come across.

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