Glossary Terms for the Letter K View the Entire Glossary A  | B  | C  | D  | E  | F  | G  | H  | I  | J  | K  | L  | M  | P  | Q  | R  | S  | T  | U  | V  | W Keystone Originally, the final supporting unit at the center of an arch. It is generally wedge shaped. At Stone Legends, Keystones are individual Units, as well as being used in other Units.