Vocal music is a term describing classic mainstream American pop performed either by individual singers or vocal groups. Before rock & roll, pop vocalists didn't write their own material, they interpreted songs written by such professional songwriters as Irving Berlin, George Gershwin and Cole Porter. Often, they would sing these songs supported by an orchestra or a swinging big band. During the '30s and '40s, vocalists were the centerpiece of these big bands and such singers as Bing Crosby and Frank Sinatra became huge stars, as did such groups as the Andrews Sisters. These vocal groups paved the way for the vocal groups of the '40s and '50s, who sang similar songs in similar styles, but tended toward lighter material. For the remainder of the century, vocalists continued to return to the traditional pop and showtunes for inspiration, and they often built upon the vocal innovations of such singers as Crosby, Sinatra, Tony Bennett, Ella Fitzgerald and Mel Torme.