The American popular song form derives from a long history of European folk song, theater music, and light opera, and was modified in America by Broadway musicals, African American folk songs, the blues, and other musics. The most common popular song forms played in jazz are of the AABA, 32-bar type, the 32-bars divided into eight-bar phrases ("The Man I Love" or "I Got Rhythm" are typical).
The B section of these songs is called the bridge (or the release, or the channel), and its words, melody, and harmony contrast to the A sections. Popular song forms can also be divided into 4-bar phrases, and they may be 16-, 32-, or 64-bars long. Other forms also exist, such as AABA and ABAC. In earlier times popular songs were divided into two parts, the verse (whose words set the scene for what was to come and whose melody was freer and nonrepetitive) and the refrain (whose words and melody were usually repeated with small variations in every chorus. By the late 1920s the verse had begun to be dropped in performance, and by the 1940s it was not even written as part of the song, turning songs into vignettes whose titles were often repeated as a "hook" ("Blue Moon," for example, or "It Might as Well Be Spring"), leaving only the refrain. The words, melody, and harmonic structure of popular songs all function together to continually recycle back to the beginning to repeat again. (The fade-out at the end of many recordings of popular songs is recognition that the form is capable of being repeated forever.)