sandwich board vs signboard

sandwich board and signboard both are nouns.

Word	Noun	Adjective	Verb	Adverb
sandwich board	Yes	No	No	No
signboard	Yes	No	No	No

As nouns, signboard is a hypernym of sandwich board; that is, signboard is a word with a broader meaning than sandwich board:

sandwich board: signboard consisting of two hinged boards that hang front and back from the shoulders of a walker and are used to display advertisements
signboard: structure displaying a board on which advertisements can be posted

Other hypernyms of sandwich board include sign.

sandwich board (noun)	signboard (noun)
signboard consisting of two hinged boards that hang front and back from the shoulders of a walker and are used to display advertisements	structure displaying a board on which advertisements can be posted