rug pad vs carpet pad

rug pad and carpet pad both are nouns.

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

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

Other synonyms of rug pad include underlay, underlayment.

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