TECNIFIBRE X-One Bi-phase 17G Natural Tennis String Reel Tecnifibre's X-One Biphase uses a heating process (H2C) which increases the cohesion between molecules allowing this string to have excellent power and exceptional feel. Gauge: 17 / 1.24 mm Composition: Multifilament - Polyurethane - H2C - Biphasic process Length: 660 ft (200 m)