A-melo, B-melo, and sabi are the section labels widely used in Japanese pop. In general the A-melo sets out the story or situation, the B-melo builds anticipation, and the sabi delivers the most memorable melody and words. These are not fixed rules, though, and you can change the length and order to suit the song.
A-melo: laying the foundation
The A-melo, the part closest to a verse, usually carries the situation in the lyrics and the atmosphere of the song. Keeping the range and the accompaniment restrained leaves room for the build that follows. Even when the same melody repeats, changes in the lyrics and the arrangement can move the story forward.
B-melo: the run-up to the chorus
The B-melo is the bridge from the A-melo to the sabi. Changing the chord progression, pushing the melody upward, or increasing rhythmic density creates the sense that the sabi is coming. It is not always necessary, but it is effective when you want the sabi to feel big.
Sabi: what you most want to say
The sabi is the central section where you place the song’s hook melody and words. Widening the range, thickening the accompaniment, and raising the volume or intensity all create contrast with the A-melo. Even a short phrase works as a sabi if it is repeated and strong enough to stick.
How these relate to English terms
In pop outside Japan, the part closest to the A-melo is called the verse, the part closest to the B-melo the pre-chorus, and the sabi the chorus or hook. The correspondence is not exact from song to song, so listen for what each section does rather than relying on the name.
Visualizing the structure in your DAW
Placing markers in the arrange window and naming them “A1”, “B1”, “Chorus 1” and so on makes the overall length and the repeats easy to see. If you get stuck while writing, try listing what you add and what you take away in each section.
