pillar box vs Britain

pillar box and Britain both are nouns.

Word	Noun	Adjective	Verb	Adverb
pillar box	Yes	No	No	No
Britain	Yes	No	No	No

pillar box (noun)	Britain (noun)
a red pillar-shaped letter box	a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom