There are two distinct uses of the word "chorus." In the thirty-two bar song form that was most common in the earlier twentieth-century popular music (especially the Tin Pan Alley tradition), "chorus" referred to the entire main section of the song (which was in a thirty-two bar AABA form). Beginning in the rock music of the 1950s, another form became more common in commercial pop music, which was based in an open-ended cycle of verses instead of a fixed 32-bar form. In this form (which is more common than thirty-two bar form in later-twentieth century pop music), "choruses" with fixed lyrics are alternated with "verses" in which the lyrics are different with each repetition. In this use of the word, chorus contrasts with the verse, which usually has a sense of leading up to the chorus. "Many popular songs, particularly from early in this century, are in a verse and a chorus ("refrain") form. Most popular songs from the middle of the century consist only of a chorus."