There are mainly two types of tennis strings - natural gut and synthetic strings. Originally natural gut strings were used to string tennis rackets. In the beginning they were made from the outer skin of sheep intestine and eventually cows started being used for this. Natural gut strings are considered to have more "feel" than synthetic strings but tend to cost more as well compared to the economical synthetic string alternatives. Synthetic strings are mainly made from nylon, polyester and kevlar, and they are used either individually or in a combination. Generally nylon is the preferred material for synthetic strings but polyester and kevlar are used to increase durability. Among these, natural gut is the most elastic(which means it will stretch the most for a given load) and kevlar is the stiffest. The stiffness for nylon and polyester strings lies between that of gut and kevlar. Generally stiffer strings will generate greater impact forces.
Generally lower string tension results in more power but lesser control and higher string tension results in lesser power but more control. Tennis racket manufacturers usually have a string tension range mentioned for the racket and one way to go about things could be to start in the middle of the range and go higher or lower based on whether you need more power or not.
| European gauge | US gauge | Inches | mm |
|---|---|---|---|
| 12 | 13 | 0.065 - 0.071 | 1.65 - 1.80 |
| 11 | 14 | 0.059 - 0.065 | 1.50 - 1.65 |
| 9.5 | 15 | 0.056 - 0.059 | 1.41 - 1.49 |
| 9 | 15L | 0.052 - 0.056 | 1.33 - 1.41 |
| 8.5 | 16 | 0.050 - 0.053 | 1.26 - 1.34 |
| 8 | 16L | 0.048 - 0.051 | 1.22 - 1.30 |
| 7.5 | 17 | 0.046 - 0.049 | 1.16 - 1.24 |
| 7 | 18 | 0.042 - 0.046 | 1.06 - 1.16 |
| 4 | 19 | 0.035 - 0.042 | 0.90 - 1.06 |
| 3.5 | 20 | 0.031 - 0.035 | 0.80 - 0.90 |
| 3 | 21 | 0.028 - 0.031 | 0.70 - 0.80 |
| 2.5 | 22 | 0.024 - 0.028 | 0.60 - 0.70 |
