Musical form — how a piece is built
Six ways of building, all on one melody: what repeats, what returns, what returns changed — and what comes back in a different key.
A piece is more than a run of notes. It has a build: something returns, something stands between the returns, something comes back altered. That build is called form, and it is the reason a piece of three minutes does not sound like three minutes of accident.
This page carries the same plain melody six times over, built differently each time. What changes is not the notes but their arrangement — and that is exactly what can be counted.
The pictures are engraved and measured: the rehearsal letters above the sections, the double barlines between them and the run of notes in each section are in the picture and in the sound, and the two can be held against each other.
The ostinato
A short pattern runs underneath, and it runs the same way every time. Above it the melody moves on freely. The pattern does not change, it only stops — and as long as it runs, it carries everything above it. Anyone who knows bass figures in pop songs knows the ostinato.
The canon
The same melody twice, but not at the same time: the second voice starts later and then sings the same thing note for note. What sounds together was never separately invented — it comes about because one single melody accompanies itself.
Song form
Three sections, and the first comes back unchanged at the end. Something else stands in between, so that the return can be heard as a return at all. The simplest build that sounds like a whole, and for that reason the most common one there is.
The rondo
The same section returns several times, and each time something new stands between its appearances. What returns holds the piece together, what stands between keeps it moving. The more often the refrain comes, the more the listener is already waiting for it.
Theme and variation
The theme stands there plainly at the start. Then it comes back — ornamented, circled around, broken into smaller values. The corner notes stay the same, otherwise it would no longer be the theme; everything between them is new. That is why the second section is called A with a prime mark.
Sonata form
Two themes, and at first they stand in different keys: the first in the home key, the second higher. In the middle both are worked over. At the end both return, and now the second one also stands in the home key — that settlement is the whole point of the form.
The six forms at a glance
Every row names the build and what it is recognised by in the picture. The letter sequences stand exactly like that above the pictures on this page; they are counted rather than guessed.
| Form | Build | What you recognise it by |
|---|---|---|
| The ostinato | one pattern, running throughout | The lower voice plays the same run of notes three times |
| The canon | one melody, twice, offset | The second voice repeats the first, two beats later |
| Song form | A B A | The first and the third section sound alike note for note |
| The rondo | A B A C A | The same section sounds three times, unchanged |
| Theme and variation | A A′ | Every fourth note of the variation is a note of the theme |
| Sonata form | A B C A B | The second theme sounds higher at first and in the home key at the end |
How a form is read
Sections get letters: the first is called A, the next different one B, the one after that C. When a section returns, it gets its old letter back — which is why song form reads as A B A and the rondo as A B A C A. When it returns altered, it gets a prime mark.
In the score a double barline separates the sections, and a thick final barline closes the piece. Neither is decoration: the double barline says that something new begins here without the music stopping.
Two of the six forms carry no letters, because they have no sections. In the ostinato the same pattern runs on unchanged while something else moves above it; in the canon the second voice sings the same melody, only later. Their build stands on top of itself rather than one part after another.
What stands next to it
How the lines turn into notes: How to read sheet music. The strictest form of all, with six transformations of one theme: Fugue. How a single bar is counted, before counting whole sections: Time signatures.
Music symbols · Music notation glossary
Anotona does this for you
Photograph a page of sheet music or play the melody — Anotona finds the key, engraves the notes and transposes them into any other. Free.
See AnotonaEvery note on this page is computed, not typed in — following the rule that also makes F# major spell its seventh degree E#, not F.