Info from Manufacturer: Phosphor Bronze was introduced to string making by D'Addario in 1974 and has become synonymous with warm, bright, and well balanced acoustic tone. D'Addario Phosphor Bronze strings are precision wound with corrosion resistant phosphor bronze onto a carefully drawn, hexagonally shaped, high carbon steel core. The result is long lasting, bright sounding tone with excellent intonation.
.020 gauge phosphor bronze wound acoustic guitar string. Preferred for its warm, bright, and well balanced acoustic tone. Perfect as a replacement string or for creating customized sets. 

Made in the U.S.A. for the highest quality and performance.