Cusp and Keystone

Cusp noun - An interval of time just before the onset of something.

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

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

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