The invention relates to a key (1) for locking a cylinder lock, comprising at least one first surface (2) and at least one second surface (2') and with sensing positions (4) provided along the length of the key (1). At least one coding (3) is provided on the first surface (2), and at least one additional coding (3') is provided on the second surface (2'). The codings comprise control locations (6, 6', 7, 7') which are provided at the sensing positions (4) transversely to the length of the key (1) and which are arranged at specific control heights (5). At least one control location (25, 25') on both surfaces (2, 2') has a different dimension, preferably a larger diameter, at at least one sensing position (4) than a control location (6, 6', 7, 7') at another sensing position (4). The invention also relates to a lock for such a key and to a system consisting of at least two locks and at least one key.