A Wedge is a very shallow sloped triangular shape that can be pushed into a small gap to enlarge it. As the sloped side moves deeper into the crack it levers the gap apart.

It must be made of a strong material, as the forces transmitted can be VERY large, and the surfaces of the wedge can become damaged.

The smoothness of the wedge faces and the strength of the contact points around the gap, make a massive difference to the effectiveness of this tool.