Compound meters: 6/8, 9/8, 12/8

In simple time the beat divides into two. In compound time it naturally divides into three: the beat is a dotted quarter (= 3 eighths). This is the rhythm that rocks a cradle, gallops, and dances the jig.

A familiar example

The nursery rhyme Row, Row, Row Your Boat is a textbook 6/8: two big beats per bar, each divided in three.

M:6/8
L:1/8
Q:3/8=60
K:C
C3 C3 | C2 D E3 | E2 D E2 F | G6 | ccc GGG | EEE CCC | G2 F E2 D | C6 |]
w: Row, row, row your boat gent-ly down the stream mer-ri-ly mer-ri-ly mer-ri-ly mer-ri-ly life is but a dream

Count: "1-la-li 2-la-li" or "1 & a 2 & a" (or "1 2 3 4 5 6" at a slow tempo). Notice the quarter + eighth pattern ("1–a") that creates the characteristic lilt: in 6/8 it is the equivalent of a long-short triplet rhythm.

6/8 or 3/4?

Both contain six eighths, but not the same pulse: 3/4 = 3 beats of two eighths (1 2 | 3 4 | 5 6); 6/8 = 2 beats of three eighths (1 2 3 | 4 5 6). Switching between the two feels creates a hemiola, a favourite effect of the Baroque era (at the end of courantes and minuets) and of Latin American music ("America" from West Side Story is a famous example).

12/8: the ballad and the slow blues

M:12/8
L:1/8
Q:3/8=50
K:C
"C"CEG cGE CEG cGE | "F"FAc fcA "G"GBd gdB | "C"c12 |]

9/8 is rarer: Bach's Jesu, Joy of Man's Desiring is often written that way, with its continuous stream of eighths grouped in threes.