Compound meter: 6/8, 9/8 and 12/8

Six eighth notes can be counted as three pairs or two triplets; 6/8 means two beats of three, the rocking feel of Row, Row, Row Your Boat.

Beats that split into three

Sing "Row, row, row your boat" and tap your foot. You will probably tap twice per line of words, not six times, and each tap falls on a group of three quick syllables: "mer-ri-ly, mer-ri-ly". That is compound meter: the beat divides into three equal parts instead of two. Because three eighth notes add up to a dotted quarter, the beat in 6/8 is a dotted quarter note. Every compound meter has a dotted note as its beat, which is the easiest way to recognise one.

Compound meters group their beats exactly like simple ones. Two beats per bar is compound duple, three is compound triple and four is compound quadruple. The time signatures are 6/8, 9/8 and 12/8, and here the top number does not count beats directly. It counts the divisions: 6/8 has six eighth notes in a bar, grouped into two beats of three. Divide the top number by three to get the beats: 6 ÷ 3 = 2, 9 ÷ 3 = 3, 12 ÷ 3 = 4. Any time signature with 6, 9 or 12 on top is compound.

6/8 is not 3/4

A bar of 6/8 and a bar of 3/4 hold the same total length, six eighth notes. The difference is the grouping. In 3/4 the six eighths form three beats of two: 1 & 2 & 3 &. In 6/8 they form two beats of three: 1 & a 2 & a. The beams in written music show which one you are reading, since they join the notes that belong to one beat. Clap six even claps and accent claps 1, 3 and 5, then accent only 1 and 4: you have just played 3/4 and then 6/8 with exactly the same notes. The music theory word for a passage that switches between these two groupings is a hemiola.

Tempo in compound meter usually counts the dotted-quarter beat. At 60 dotted-quarter beats per minute, each beat lasts one second and each eighth note a third of a second, about 0.333 seconds. If you set the metronome to count eighth notes instead, you would need 180 clicks a minute for the same speed, and the music would lose its swaying two-beat feel. In the lab, choose 6/8 and turn on Divisions: you will hear a strong click, two soft ones, a medium click, two soft ones.

Row, Row, Row Your Boat

The words of this round were printed in 1852, and the tune now sung with them was printed in 1881 and credited to Eliphalet Oram Lyte, so the melody is well out of copyright. It is a near-perfect 6/8 example. "Row, row" is two dotted quarters, one per beat. "Row your boat" is a quarter, an eighth and a dotted quarter, the long-short pattern that fills one compound beat with 2 + 1 eighths. "Merrily, merrily, merrily, merrily" is twelve eighth notes, four full beats of three. Tap the whole round in the lab below at 60 dotted-quarter beats per minute.