What materials do they use to make harp strings?
Harp strings come in many varieties. The low strings which produce the bass notes are often wire-wound and similar to piano or guitar strings. The high strings in the top octaves of the harp are often made of nylon and appear white/translucent. The strings in the middle of the harp are often made of gut (yes, actually made from the intestines of cows) and have a fibrous as well as more opaque appearance.
