Provided is a locking device that allows a plurality of cylinder locks having different security levels to be locked and unlocked with the same unlocking key. A locking device comprising: a cylinder lock having an outer cylinder, and an inner cylinder that is rotatably fit with the outer cylinder; and an unlocking key corresponding to a key hole formed in the inner cylinder, wherein a projecting part is formed at the back end of the key hole, and a positioning recess part that can be fit with the projecting part is formed at the front end of the unlocking key.

Set for installing a leaf (3) of a sliding door (1) provided with a wheel set (12) in a frame (2), the set including a leaf (3), a frame (2), a wheel set (12) and an aid (1), whereby the aid (15) includes supporting elements (17) that each define a support point (20) for the leaf (3), whereby the distance (D.sub.s) between the support points (20) is less than the thickness (D.sub.v) of the leaf (3) and whereby the aid (15) includes at least one connecting element (16) to fix the supporting elements (17) with respect one another.