back saw vs carpenter's saw

back saw and carpenter's saw both are nouns.

Word	Noun	Adjective	Verb	Adverb
back saw	Yes	No	No	No
carpenter's saw	Yes	No	No	No

As nouns, carpenter's saw is a hypernym of back saw; that is, carpenter's saw is a word with a broader meaning than back saw:

back saw: a handsaw that is stiffened by metal reinforcement along the upper edge
carpenter's saw: a saw used with one hand for cutting wood

Other hypernyms of back saw include hand saw, handsaw.

back saw (noun)	carpenter's saw (noun)
a handsaw that is stiffened by metal reinforcement along the upper edge	a saw used with one hand for cutting wood