 Doo-wop's style is a mixture of precedents in composition, orchestration, and vocals that figured in American popular music created by song writers and vocal groups, both black and white, from the 1930s to the 1940s.Such composers as Rodgers and Hart (in their 1934 song "Blue Moon"), and Hoagy Carmichael and Frank Loesser (in their 1938 "Heart and Soul") used a I-vi-ii-V-loop chord progression in those hit songs; composers of doo-wop songs varied this slightly but significantly to the chord progression I–vi–IV–V, so influential that it is sometimes referred to as the 50s progression.This characteristic harmonic layout was combined with the AABA chorus form typical for Tin Pan Alley songs.