Among the most popular of D'Addario's acoustic guitar strings are those round wound in phosphor bronze. Slightly mellower than 80/20 bronze, phosphor bronze wound strings are synonymous with warm, bright, and well balanced acoustic tone. Made with corrosion resistant metal wrapped on a steel hex core, these strings will impress you with their longevity and superior tonal characteristics. D'Addario was the first to make these strings, beginning in 1974, and their popularity was immediate and long lasting. The EXP range feature D'Addario's patented life extending coating. A polymer coating is applied to the wrap wire BEFORE it is wound on a steel core, resulting in strings that sound and feel just like uncoated strings, while lasting at least four times longer.