Rails are like big fins - they are what you rely on when leaning the board on edge with all of your power. Their purpose is to provide you with the right amount of resistance and bite through turns, and recovery out of maneuvers. The rail consists of a bottom and a top edge that fuse together at the ball or outer most point. A tucked bottom edge yields a looser more forgiving feeling, whereas a hard, angular bottom edge produces an edgy, grabby feeling. In general, surfboards have soft leading rails and hard edges off the fins and out the tail. The bottom edge is the most critical part of the rail – the type of bottom contours you use generally dictates the flow of the bottom edges.