This article quotes generously from the excellent site Stringforum.
The string market is a jungle. There are countless of brands, materials, and suppliers who work hard to get on the racket of top players as well as amateurs. You can divide the different kinds of strings in either natural (gut) or synthetic strings.
Natural gut strings are made of cows’ gut in a complex process which renders them far more expensive than synthetic strings. They are lively and elastic strings, but sensitive to weather conditions. There are still professional players who use natural gut in (usually half) the string bed, but for amateur and intermediate players it’s not a common option due to their cost and sensitivity. They break easily which makes them even more expensive and you would need to re-string your rackets quite often.
Synthetic strings are the name of the game these days and here you have a variety of different types and characteristics. Here are the main types of synthetic strings.
The most frequently used string type. Nylon strings are normally made of a single nylon core and various resistant wraps. Due to its excellent dynamic properties nylon (polyamide) is well suited as a material for tennis strings. The high number of different types of constructions (wrap material and wrap angle) influence the string’s playing characteristics significantly. As a rule of thumb, nylon strings with multiple wraps can be considered higher grade than single wrap nylon strings. The wraps reduce the tension loss usually experienced with nylon strings. Nylon strings are suitable for players who have a normal or high string consumption.
Polyester strings show a fairly simple structure: they consist of a single polyester fiber with a thin coating. This type of construction is termed “monofilament”. They come in different gauges (1.10-1.35mm) which enables you to choose among different elasticity/durability levels. Polyester strings feel quite stiff compared to nylon or multifilament strings, but on the other hand they provide significantly better durability, allowing for thinner gauges. Polyester strings are only recommendable for players with high string consumption. For these players, polyester strings offer a great price/performance ratio.
To bring synthetic strings’ playability more into line with natural gut, many microfibers (which can be of many different materials) are twisted together to a string, which is wrapped with a resistant cover. This means you get higher elasticity and better playability. The problem is that multifilament strings tend to break soon once the outer wrap is damaged (the strings “fray”). Also these strings cost more than nylon strings because of the complex manufacturing process.
Structured (textured) strings are designed to provide better ball bite and thus enhanced spin (it seems to be all about spin these days!) Most of these strings indeed offer great spin potential and in line with that better control, but unfortunately the texture usually wears within a short time and the strings become smooth. Another downside is their decreased durability.
Hybrid strings are a combination of two different strings for mains and crosses. In a uniformly strung racquet it’s almost always a main string that breaks. This is because the main strings move a lot more than the cross strings so the cross strings “saw” into the main strings, causing notches and eventually breakage. That’s why in hybrid strings usually a durable string is used as the main string (e.g. polyester or aramid/kevlar/technora). As cross strings usually highly elastic synthetic strings or natural gut strings are used to provide comfort and feel. Hybrids provide good playing characteristics while a poly/multi hybrid often lasts longer than a pure poly or pure multifilament string job.
Some general stuff about strings
To get the best out of your racquet you’ll have to do a little more than just use the best string. The choice of the right tension is about as important as the choice of the racquet frame. As a general rule: the harder you string the less power you get and the more control you have. With lower tension you gain more power but also lose control. In any case you should try different tensions; if you play better – great, and if you don’t, you can get back to the old tension the next time. To show you the effects different string tensions and diameters can have on your racquet’s performance I created following tables:
Strings lose elasticity with time, one type of string faster, another type of string slower. This has a negative effect on the playability; players with a sensitive arm will feel it soon. In general you shouldn’t play a string longer than 2 to 3 months. Then it’s about time to cut out the strings and restring your racquet.
Often the diameter of a string is not given in millimeters but in the old “gauge”. Following table helps you convert between these two measures: