it playability that is more similar to natural gut than, arguably, other synthetic materials. It also has low overall stiffness. The ProKennex IQ Element Z string, for instance, has the lowest stiffness of any synthetic string yet tested. The drawback of Zyex is that the outer wrapping materials in Zyex tennis string tend to be much less durable than the Zyex filaments inside the string and do not bond with them. This can lead to the outer wrapping wearing away, leaving the inner Zyex filaments. Polyolefin Polyolefin is one of the softest synthetic string materials, rivaling some Zyex and most nylon multi-filaments. It offers mediocre durability and tension retention, so it is generally used as the cross string in a hybrid string setup. For those who do not break strings very quickly, however, 100% polyolefin stringing may be a good alternative to natural gut and multi-filament strings. This is especially true for those who have had tennis elbow and find natural gut string to be too costly. Metal wire Metal wire, usually piano wire, was used in some historical racquets. Titanium Titanium is also used. Usually Titanium strings are based on Nylon or multi-filament strings with the titanium applied with the coating of the string, to protect the material from UV and abrasion, or is integrated into the filaments to modify the play-ability of the string. Based on Nylon or multi-filament strings, the titanium is either applied with the coating of the string, protecting the material from UV radiation and abrasion, or the titanium is integrated into the filaments to modify the play-ability of the string. Titanium strings are no longer popular today. Gauge The "gauge" number determines the thickness of the string. A string rated with a high gauge number is a thinner string, and vice versa. Thinner strings typically offer higher performance but break more frequently than thicker strings. Stringing pattern Virtually all modern racquets are strung in a criss-cross horizontal-vertical pattern. Various other patterns have been used in history with varying success. Double strung tennis racquets were introduced in 1977 but the "spaghetti racquets" were later banned because they permitted excessive spin and were too successful. Racquets are strung either with two separate strings (and thus four knots), or with a single string (resulting in only two knots). Sometimes, a hybrid of two different string types may be used in the same racquet. Traditionally, a double half