Abstract: The lock is preferably to be used for burglar alarms for marking articles for sale, and it comprises two members automatically interengaging when the first member is inserted into a cavity in the second member, a loose third member being positioned in said cavity. The locking effect is provided by the loose third member comprising one, optionally more inclined surfaces capable of being tangent to and sliding along a corresponding inner wall in the cavity of the second member. The cavity of the second member is pointed upwards, i.e., towards the inlet opening. A spring is provided below the loose third member, and this spring presses as a consequence of the inclined wall the third member upwards and towards the first member.