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.