 Hit songs by black groups such as the Ink Spots ("If I Didn't Care", one of the best selling singles worldwide of all time, and "Address Unknown") and the Mills Brothers ("Paper Doll", "You Always Hurt The One You Love" and "Glow Worm") were generally slow songs in swing time with simple instrumentation.Doo-wop street singers generally performed without instrumentation, but made their musical style distinctive, whether using fast or slow tempos, by keeping time with a swing-like off-beat, while using the "doo-wop" syllables as a substitute for drums and a bass vocalist as a substitute for a bass instrument.Doo-wop's characteristic vocal style was influenced by groups such as the Mills Brothers, whose close four-part harmony derived from the vocal harmonies of the earlier barbershop quartet.