Abstract: A lock includes a hollow casing, a core rotatably received in the hollow casing, a key detachably inserted into the core and having a pattern on both sides of the key, locking plates movably received in the slits and at least one of the locking plates being extending out of a corresponding slit and into at least one elongated hole to interfere rotation of the core and a positioning plate received in the hollow casing and being movable with respect to the core. Movement of the positioning plate abuts the locking plates to secure movement of the locking plates, such that only the locking plates are retracted into the core by the pattern of the key to avoid interference with the rotation of the core, whereby the core is immovable.