Tecnifibre Black Code 4S is a co-polyester tennis string with a square shape and four spin-friendly edges. For our playtest team this string's most undeniable characteristic was control. Put simply, Black Code 4S enabled us to execute our most powerful strokes without fear of overhitting. Our ability to take massive upward cuts at the ball -- especially when combined with this string's four grippy edges -- was a recipe for massive spin. Another prominent feature of this very spin-friendly co-poly was its firm feel.