The seal (9) fits in a recess (14) in one (13) of at least two components (13, 15) which are to be sealingly connected. It includes a projection (11) engaging a wall of the recess and is only thereby held in its sealing position. The seal may have a trapezium-shaped cross-section and may be used for sealing the lens (15) and/or the protective housing of a motor vehicle head lamp, a projection (11) being provided on each of the two opposite sides and extending in a direction at right angles to the direction (12) in which it is inserted.