Cover wrap alloy composed of 80% Copper and 20% Zinc. Sometimes referred to as "brass". Designed for a brighter and more crisp sound out of the pack.Only SIT offers the STAY-IN-TUNE Guarantee - "If nothing is causing slippage on your instrument, the S.I.T. Strings Corp. guarantees your strings to Stay-In-Tune and provide unsurpassed brilliance and bendability."