rules regarding rackets have changed over time, as material and engineering advances have been made. For example, the maximum length of the frame had been until 1997, when it was shortened to . Many companies manufacture and distribute tennis rackets. Wilson, Head and Babolat are three of the most commonly used brands; however, many more companies exist. The same companies sponsor players to use these rackets in the hopes that the company name will become better known by the public. Strings There are multiple types of tennis strings, including natural gut and synthetic stings made from materials such as nylon, kevlar, or polyester Natural gut The first type of tennis strings available were natural gut strings, introduced by Babolat. They were the only type used until synthetic strings were introduced in the 1950s. Natural gut strings are still used frequently by players such as Roger Federer. They are made from cow intestines, and provide increased power, and are easier on the arm than most strings. Synthetic Most synthetic strings are made from monofilament or multifiliament nylon strings. Monofilament strings are cheap to buy, and are used widely by many recreational level players for their all round performance, while multifilament strings are created to mimic natural gut more closely by weaving together fibres, but are generally more expensive than their monofilament counterparts. Polyester strings allow for more spin on the ball than any other string, due to their firm strings, while keeping control of the ball, and this is why many players use them, especially higher player ones. Kevlar tennis strings are highly durable, and are mostly used by players that frequently break strings, because they maintain tension well, but these strings can be stiff on the arm. Hybrid strings Hybrid stringing is when a tennis racket is strung with two different strings for the mains (the vertical strings) and the crosses (the horizontal strings). This is most commonly done with two different strings that are made of different materials, but can also be done with two different types of the same string. A notable example of a player using hybrid strings is Roger Federer, using natural gut strings in his mains and polyester strings in his crosses. Balls Tennis balls were originally made of cloth strips stitched together with thread and stuffed with feathers. Modern tennis balls are made of hollow vulcanized rubber with a felt coating. Traditionally white, the