English: This is a drawing of Saint Peter's keys, a device to lift stone blocks. It is also known as a "Roman three-legged lewis", "three-legged lewis", "Wilson bolt", and "dovetailed lewis".

The entire thing fits into a dovetailed seating in the top of a building stone. It is made from three pieces of rectangular-section 13 mm-thick steel (legs) held together with a shackle, allowing connection to a lifting hook. The middle leg is square throughout its length. The outer legs are thinner at the top, flaring towards the bottom. Held together, the three legs form a dovetail shape. The lewis hole seating is undercut (similar to a chain-linked lewis hole) to match its profile.

The first outer leg is inserted into the lewis hole, followed by the second outer leg. The inner (parallel) leg is inserted last, pushing the outer legs into contact with the inside of the lewis hole. The shackle is unbolted, placed over the legs, and the bolt fastened through both the shackle eyes and the eye in the top of each leg.