Gravestone and Keystone

Gravestone noun - A shaped stone laid over or erected near a grave and usually bearing an inscription to identify and preserve the memory of the deceased.

Keystone and gravestone are semantically related. In some cases you can use "Keystone" instead a noun "Gravestone".

Keystone noun - An immaterial thing upon which something else rests.

Gravestone and keystone are semantically related. Sometimes you can use "Gravestone" instead a noun "Keystone".