A seal including a body with a longitudinal axis X that includes a base-piece having a first end and an opposed second end, two opposed side walls connecting the two ends, and two shoulders located on either side of each of the two side walls and at one of the ends. The body is made of a single flexible material and, in a section perpendicular to the X axis, the width of the base-piece, from one side wall to the other, in the vicinity of the end on the opposite side from the shoulders is larger than the width of the base-piece, from one side wall to the other, in the vicinity of the end near the shoulders.