A) Rudderlike device(s) used beneath a surfboard to assist control, direction and drive. Many different fin shapes are possible, but most are designed to resemble a dolphin's dorsal fin. Today fins are mostly used in groups of three-two ahead, one behind-a configuration known as a Thruster (see Thruster). They're also used, less frequently, in pairs or singles. B) Rubber swim fins worn on the feet of body boarders and body surfers to help catch waves in deep water.