The difference has to do with the ratio of copper to zinc in the wrap wire of the Bronze and Brass strings, and with the formula for the PhosBronze and Blue Steel Acoustic Strings. For instance, VintageBronze strings have a ratio of 85% copper to 15% zinc, while Formula82/R strings have a ratio of 80% copper to 20% zinc. Phosphor bronze strings, on the other hand, such as our Blue Steel acoustic strings and PhosBronze acoustics, have a ratio of 92% copper to 8% tin. The sound differences can be drastic or subtle, depending on how the finished strings are designed and wound. And what type of guitar the strings are on different guitars respond differently to different alloys. It is a good idea to first pick your gauge, then try each of the alloys until you find the one that works perfectly for you.