Gut Core Strings
For many centuries the only kind of string available, the gut core strings are made from sheep intestine (not “cat,” as is commonly misbelieved). Used primarily by professional classical musicians, gut core cello strings reputedly have the warmest sound, complex and rich with overtones.
That said, they are also the most difficult strings to work with, by far. They take at least a week to “settle in” once they are put on the cello, and even then are plagued with problems. Even under normal conditions, they go out of tune frequently. Variances in temperature, humidity, and other factors make this condition even more acute. And, they don’t last as long as either of the other types of strings.
Additionally, a gut core string’s response is slower than other strings, and they are “softer,” making sound a bit more difficult for a cellist to produce.