Medium Gauge (13 - 56). Developed and approved by our team of Master Luthiers, Gibson Coated Phosphor Bronze Strings are factory specified at the Gibson Acoustic facility in Bozeman, MT. These strings feature a high-carbon steel, hex-shaped core wrapped in phosphor bronze that is very thinly coated with a proprietary polymer. The result is unmatched durability that does not compromise vibrant tone like the rest. Discover the lush clarity and superior warmth that is suitable for any playing style.