The stone which completes an arch . It is at the top in the middle, usually given some prominence because of its importance. When a stone or brick arch is being built a wooden framework is first set up, on which the blocks are laid, starting at the abutments.

When the top is reached and the keystone fitted the framework may be removed. Were the keystone to be removed an arch would collapse. Hence its importance. However, not all keystones are what they seem. Sometimes a purely decorative stone is fitted to an arch that owes its support to some other form of construction. The keystone, real or decorative, is a stone wider at the top than at the bottom and is easily recognised. Many arches have false keystones, which are purely decorative, sometimes in metal. They add pleasing touch to the crown of an arch.

You can see the drawing.