Natural Gut: comes from the cow. specifically, the part of its intestine called the serosa. It is produced by drying the serosa fibers which contains collagen designed to withstand the stretching and contraction of the intestine. It is this elasticity that makes the fibers useful for tennis string. It is also the first string to be used in a tennis racquet invented by Babolat in 1875! Sometimes it’s called CAT gut. But it’s not made from cats so please stop spreading this misinformation. There is also a much cheaper natural gut that comes from India that IS made of Sheep gut. It’s not worth the cheaper price. I’ve tried it. They soak it in rose oil or some cheap oils but that still doesn’t prevent it from breaking prematurely.

Synthetic gut/Nylon: They’ve given it a fancy name of, “synthetic gut.” However these are all made of nylon. DuPont pioneered a lot of the plastics as early as the 1930s but it wasn’t implemented into tennis until the 50s. Typically manufactured with a monofilament core and a layer of outer wrap. However I do not call it “synthetic gut” because I believe it is a misleading term. I call it what it is, nylon. It can really mimick natural gut but only at very high tensions. 60+. It’s what allowed tennis to become more accessible to the general public.  However, now that modern racquets are super stiff and powerful so nylons have been eclipsed by polyesters because the industry tries to apply it in the same way as they would in a wooden racquet.

Multifilament: these are also mostly made of nylons but I don’t call it a nylon because the manufactured structure of these can be quite complex. They can be a composed of hundreds or thousands of different filaments/fibers that are usually bonded or twisted together. Various materials are added to change the properties of the string but the result is typically a softer string. Mostly in attempt to mimic the properties of natural gut but honestly they don’t come close.

Monofilament: a solid single extruded untwisted synthetic filament. Typically made of polyester or PEEK (polyetheretherketone) and there are 2 companies that make it out of nylon.

Polyester: they are made of exactly that, polyester. it was a revolutionary string when introduced in 1997 and the entire sport hasn’t looked back since. They claim polyester is more durable but that can be misleading. They may not notch and break as quickly as other strings but their usable lifespan is quite short. Some professionals don’t use them for more than 7 games in a match because they hit the ball so hard, effectively stretching the spring out of them. More recently they’ve rebranded them as “co-poly”. They claim since they mix so many other ingredients into it they can no longer simply call it, “polyester”. But don’t be fooled, it is still polyester.

Kevlar/Aramid: It’s also what bulletproof vests are made of. It is a synthetic fiber that is heat resistant and has tensile strength stronger than steel. When it is manufactured into tennis string, hundreds or thousands of fibers are twisted or braided to create the string. Almost effectively making it a multifilament but with almost unbreakable properties. Because of it’s enormously high tensile strength, it is also a VERY stiff string. Much stiffer than polyester however, when paired with a soft cross string and strung at low tensions, it has been proven to be a very high performing string. Andre Agassi was the first to use kevlar/aramid strings on tour in the 90s. He also popularized the mullet…