Approx. 10 meters of string are strung onto an average tennis racquet head – almost all of them with a string diameter (gauge) between 1.2 and 1.3 mm. Basically you can say: Thinner strings offer more playing comfort and joint protection, thicker strings tear less quickly and offer more control on hard hits.
But the playing characteristics of a tennis string are not only determined by its diameter. Material and construction have at least as much influence.
Natural gut strings are mostly made from the gut of cows. Natural gut strings are still the measure of all things when it comes to power, feeling, comfort and maintaining elasticity. But gut strings are also very expensive and delicate. To make them more resistant to friction and moisture, their fiber structures are often plasticized or coated.
Simple polyamide strings (nylon strings), often called “synthetic gut”, are the most economical alternative to natural gut – but without outstanding strengths. They usually consist of a solid core with a single-layer filament sheath.
Multifilament strings, on the other hand, are complex constructions often made of more than 1,000 individual fibers (filaments). Their various, often very high-quality materials get merged – or braided – into one piece. The main advantages of a multifilament string derive from its very good, long-lasting elasticity: maximum shock absorption (comfort), high playing comfort and a lot of touch for a variable game. For very powerful players, however, multifilament strings also have the disadvantage that they tear faster than, for example, monofilament strings. Tournament players therefore actually only use multifilament strings for their cross strings in a hybrid string setup.
Monofilament strings have a rather simple design. A certain string material, or a combination of materials, is extruded, or drawn through a geometrically shaped dye, to form a solid piece of string. The materials used can be polyester, copolymers ("soft polyester"), aramid (“Kevlar”) or PEEK polymers. This basic construction is rather tear-resistant and inelastic. Initially this provides a very "crisp" feel, which tends to decline rather quickly.
The advantages of monofilament strings are particularly relevant for “hard hitters” (tear resistance and control) and for extreme topspin players (little string slippage and good snapback effect). A special coating on the exterior or a structured surface often support their spin generating property.
On the other hand monofilaments generally transfer more shock down the racquet into the handle than a softer, multifilament string. That’s why some players only use monofilament strings for the main string in a hybrid string setup.
A hybrid string setup is the use of two different strings in the mains and cross strings of a tennis racket, e.g.:
A hybrid setup will improve the playability of many racquets as it blends the advantages of several types of strings. For many recreational players, for example, a complete set of monofilament strings is too difficult to play due to its stiffness and the lack of dwell time oft he ball on the strings.
To reduce the harsh feeling and increase playing comfort the pre-configured hybrid sets offered by the manufacturers usually couple firmer monofilament strings with softer, more forgiving strings in the crosses such as natural gut or a multifilament.