- keystone, key, headstone - the central building block at the top of an arch or vault
--1 is a kind of building block
--1 is a part of arch
--1 has particulars: quoin, coign, coigne - gravestone, headstone, tombstone - a stone that is used to mark a grave
--2 is a kind of stone; memorial, monument
--2 is a part of grave, tomb