Compound Time, Grouping & Triplets
Time signatures where each beat divides into three, how notes are grouped to show that pulse, and the triplet that borrows the same feel.
Explainer
In compound time, each main beat divides naturally into THREE, not two. 6/8 has two main beats per bar (each a dotted crotchet = 3 quavers); 9/8 has three main beats; 12/8 has four.
Notes are beamed to show this three-in-a-beat grouping clearly — three quavers sharing one beam make exactly one compound beat, the same way beaming in simple time shows where each ordinary beat falls.
A triplet borrows that same "three notes in the space of one beat" feel, but inside ordinary SIMPLE time — three notes squeezed into the space a beat's normal two quavers would take, marked with a small "3" over the group.
Both compound time and triplets exist because composers sometimes want a rolling, three-part feel to the beat even in music that's mostly built on twos.
Worked examples
6/8 — two beats per bar
Two groups of three quavers: ONE-and-a, TWO-and-a — each group of three sharing one beam.
9/8 — three beats per bar
Three groups of three quavers: ONE-and-a, TWO-and-a, THREE-and-a.
A triplet in 4/4
Three quavers marked with a small "3", squeezed into the space one crotchet beat's two quavers would normally take.