As the name would suggest, this is a natural product, literally from the gut (mainly of sheep) of animals. This type of string is still favoured by the purists for its optimum mixture of power, control and spin. It is, however the most expensive and least durable of all the strings. It is also susceptible to extremes in temperature and is especially liable to break under damp or wet conditions. There is no man made string exactly like natural gut but some of the multifilament strings come close (e.g. Tecnifibre X-One Biphase).