A rubber sliding D-Ring with outer diameter of ~50mm and thickness of ~8mm that provides a snug fit compatible with {2 in | 52 mm} webbing. Popular for sidemount diving, these are used in order to adjust trim as the cylinder buoyancy changes due to gas consumption. To move, pinch then push up and down the waist strap to desired location for the cylinder clip to compensate for the changes in tank buoyancy. Rings are sold each.