The Power Groove strings have a pure nickel cover-wrap over a hard-tempered hex core. This soft nickel alloy produces a string with great feel and intonation. Pure nickel will really bring out the tone in vintage guitars. These strings have been reformulated and re-introduced with new packaging in 2014. The exclusive S.I.T. Stay-In-Tune level wrap formula guarantees consistent tension and tone. All S.I.T. Strings have the Stay-In-Tune process at the end of the string near the ball-end to help prevent stretching and breakage.