Introduced by D'Addario in 1974, Phosphor Bronze has become the world’s most popular acoustic guitar string. Synonymous with warm and well-balanced acoustic tone, Phosphor Bronze is precision wound on a hexagonally shaped, high carbon steel core, resulting in long lasting, bright sounding tone with excellent intonation. • The world’s most popular acoustic guitar string • Warm tone • High carbon steel core and plain steel wire • Gauges ranging from Extra Light to Medium • The string gauges in this acoustic string set include: Plain Steel .009/.009, .013/.013, Phosphor Bronze Wound .021/.008(p), .029/.011(p), .036/.016(p), .045/.026(w)