RB FLATWOUND BASS FLATS authentic in every way. Constructed over a round core for increased flexibility and lower tension, the RB FLATS are wound with a pure nickel flat wire, which is then polished to a high shine, resulting in a string that is both supple and comfortable to play. With the RB FLATS, Optima have created a flat wound bass string which echo’s the classic tones of yesteryear.