britches vs knee breeches

britches and knee breeches both are nouns.

Word	Noun	Adjective	Verb	Adverb
britches	Yes	No	No	No
knee breeches	Yes	No	No	No

As nouns, knee breeches is a hypernym of britches; that is, knee breeches is a word with a broader meaning than britches:

Other hypernyms of britches include breeches, knee pants, knickerbockers, knickers.

britches (noun)	knee breeches (noun)
(used in the plural) informal term for breeches	(used in the plural) trousers ending above the knee