Luxilon ALU Power 138 is a thickest version of Luxilon's ALU line. The string improves on the first generation line of polyester strings by using a co-poly construction with aluminum. The aluminum present in ALU Power is unique to the Luxilon line, giving the string a softer feel and more playability. The co-poly construction of the string helps to retain tension and deliver a new level of elasticity never seen before from a durability string. The Luxilon line of strings has forever changed todays professional tennis. It allows players of all levels to take full cuts, creating generous amounts of spin without a loss of control. ALU Power is best suited to the player that prefers the baseline and takes fast, full swings looking to generate power and spin. Composition: Extruded Polyester Monofilament Color: Gray Gauge:15G (1.38mm) Length: 40 ft