Nickel-plated cover wrap over a hard-tempered hex core. SITs most popular and widely used electric guitar set. Great tone and longevity and suitable for almost all applications. They are also available in the widest variety of gauges and combinations. The "Stay-In-Tune" string that made us famous! Nickel-plated cover wrap over a hard-tempered hex core. Our most popular and widely used electric guitar set. Great tone and longevity and suitable for almost all applications.