A song form, specificall the, 32-bar song form, is the formal structure characteristic of hundreds (if not thousands) of popular songs, especially those composed during the first half of the twentieth century. Such songs typically divide into two sections, labeled as verse and chorus (or refrain). Most listeners think of the more familiar chorus section as the song itself, and think of the verse section as its introduction.
The chorus is usually 32 bars long, divided into four phrases of eight bars each. (The Brazilian samba and genres derived from it often use a 64-bar form, as four sections of 16 bars apiece.) The first phrase's musical material (conventionally labeled as "A") is also used for the second and fourth phrases; contrasting material ("B") is used for the third. The succession of materials over the course of the song is thus mapped as AABA.
The endings of the first, second, and fourth phrases may differ slightly, to create varying degrees of closure (saving the greatest closure for the fourth phrase). These alterations can be reflected through the use of prime marks (AA'BA', or AA'BA").