When band members are marching in one direction but want to focus their sound in another, they may rotate their bodies at the waist, so that only the upper portion of the body faces in the direction of play. This is known as "lateral marching", but is more commonly known as either "shifting", "traversing", or "sliding". A lateral march is not a change in the direction of march, only in the direction the upper body faces. Percussion players, whose large drum harnesses often prevent them from twisting their torsos, and sometimes tuba and sousaphone players, instead uses a "crab step" when moving sideways. During a crab step, the musician crosses one leg over the other, either marching on the toes or rolling the foot sideways. Percussionists may also substitute roll step when their instruments would interfere with performing the high step.