transom vs crosspiece

transom and crosspiece both are nouns.

Word	Noun	Adjective	Verb	Adverb
transom	Yes	No	No	No
crosspiece	Yes	No	No	No

As nouns, crosspiece is a hypernym of transom; that is, crosspiece is a word with a broader meaning than transom:

transom: a horizontal crosspiece across a window or separating a door from a window over it
crosspiece: a transverse brace

transom (noun)	crosspiece (noun)
a horizontal crosspiece across a window or separating a door from a window over it	a transverse brace
a window above a door that is usually hinged to a horizontal crosspiece over the door	a horizontal beam that extends across something