Abstract: The invention relates to a cap (10) for a closure device (3) designed to receive a sealing device (11), with at least two coupling parts (18, 19) joined thereto so as to be displaceable, which are spaced apart from one another in the direction of a longitudinal axis (6) and project out from an internal face (17) of a cap casing (16) in a radial direction towards the longitudinal axis (6) forming a locating region (20) between them. An internal clearance dimension (32), in particular an internal diameter (33) of the first coupling part (18) co-operating with the sealing surface (14) is between 5% and 25% smaller than an external dimension, in particular an external diameter of the sealing device (11) to be located in the region of its sealing surface (14) in the non-deformed or unclamped state. The invention further relates to a sealing device (11), a closure device (3) comprising the latter and the cap (10) as well as a container system (1).