Rudder-like device attached to the surfboard's rear bottom surface, singly or in a multifin cluster, to give directional stability, control, and maneuverability. "Surfers as a rule take fins for granted," surf writer Devon Howard noted in 2005, "But nearly every quantum leap in board design has revolved around the fin and its various configurations."

Before the development of the fin, surfers gained a small amount of traction and control through the use of convex hull shapes, and were able t...