quoin

1. Any of the corner building blocks of a building, usually larger or more ornate than the surrounding blocks.

2. The keystone of an arch.

   the keystone of an arch

3. (historical)A metal wedge which fits into the space between the type and the edge of a chase, and is tightened to fix the metal type in place.

   expandable metal or wooden wedge used by printers to lock up a form within a chase

4. A wedge of wood or iron put under the breech of heavy guns or the muzzle of siege-mortars to raise them to the proper level.

5. A number of apple varieties with a distinctive ribbed appearance, like corners of a coin.

6. (obsolete)A form of wedge used to prevent casks from moving.

regular