Vocal jazz is a form of jazz that focuses on a singer's emulation of the surrounding instruments through improvisation, scat singing, or other vocal techniques. Billie Holiday once explained "I don't feel like I'm singing, I feel like I'm playing the horn". Vocal jazz grew in conjunction with swing in the 1920s, as vocalists were often incorporated into big bands. The genre has seen transformation up to the point of the contemporary vocal jazz of today. Some of the enduring favorites of the genre include Louis Armstrong, Billie Holiday, Nina Simone, Ella Fitzgerald, Frank Sinatra, Sarah Vaughan, Diana Krall, and Tony Bennett, among many others.