Cords used for musical instruments, surgical sutures, fishing flies and the strings of tennis rackets. They are not made from cats.
Catgut varies by the tanning process (some is chromic), by varnished (a 20th century innovation) and unvarnished, and by the polish given to the surface. It is prepared in various diameters, from 0.18 millimeter to almost 5 mm.
A current supplier of catgut strings for musical instruments. Look for the video.
A vendor of gut for racquets provides an excellent page on gut for that use, including such facts as that three cows are needed for enough gut to string a racquet.
Sizes 0 through 5/0. (In the USA, for veterinary use only, though catgut is in the Pharmacopeia.)
Catgut is used for tying small flies, and is less than a millimeter in diameter.
Catgut, To Make.—Take the entrails of sheep, or any other animal, procured from the newly-killed carcass. Thoroughly clean them from all impurities and from attached fat, and wash them well in clean water; soak in soft water for two days, or in winter three days, then lay them on a table and scrape them with a small plate of copper, having a semicircular hole cut in it, the edges of which must be quite smooth and not capable of cutting. Now, after washing, put them into fresh water, and there let them remain till the next day, when they are again to be scraped. Let them soak again in water for a night, and two or three hours before they are taken out add to each gallon of water 2 oz. of pearlash. They ought now to scrape quite clean from their inner mucous coat, and will consequently be much smaller in dimensions than at first. They may now be wiped dry, slightly twisted, and passed through a hole in a piece of brass, to equalize their size; as they dry, they are passed every two or three hours through other holes, each smaller than the last. When dry they will be round and well polished, and being oiled are fit for use.
Workshop Receipts for the Use of Manufacturers, Mechanics, and Scientific Amateurs. First Series.
London: E. & F. N. Spon, 
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Last revised: 7 February 2016.