A closing/opening system for a case of a timepiece is defined particularly in first and second parts of the case. The system includes a device for producing a mechanical connection, particularly a bayonet connection between these first and second parts and a device for creating a friction torque between these first and second parts. The device for creating a friction torque includes tightening faces to cooperate with one another when closing/opening the case and which are defined in the first and second parts. The system further includes a tightening element in one of these two tightening faces. The tightening element generates a constant friction torque during a course of travel of one of the two parts relative to the other when closing/opening the case of the timepiece.