Ukulele strings used to be made out of catgut, which is actually sheep intestines. What kind of ukulele it is determines the length of the strings that you need. Now, the most common material that uke strings are made out of are: nylon. gut, and fluorocarbon. In the 1940s, the strings began being made out of nylon because it is a cheaper, more consistent, and still had a great sound. Fluorocarbon is starting to be used because it is louder, denser, brighter, and not as sensitive to temperature changes.
