Phosphor bronze (P/B) is second in popularity to the 80/20 bronze strings for acoustic guitar. They produce a bright, but slightly warmer and darker sound than bronze stirngs. The small amount of phosphorous in the alloy helps them retain their new sound longer than bronze.
The P/B string was introduced to stringmaking by D'Addario in 1974. D'Addario's are our most popular P/B sets. Most American made acoustic guitars are factory strung with Phosphor Bronze strings.