carpet pad vs rug pad

carpet pad and rug pad both are nouns.

Word	Noun	Adjective	Verb	Adverb
carpet pad	Yes	No	No	No
rug pad	Yes	No	No	No

As nouns, carpet pad and rug pad are synonyms defined as:

Other synonyms of carpet pad include underlay, underlayment.

carpet pad (noun)	rug pad (noun)
a pad placed under a carpet	a pad placed under a carpet