rug pad vs underlay

rug pad and underlay both are nouns.

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

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

Other synonyms of rug pad include carpet pad, underlayment.

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

rug pad (verb)	underlay (verb)
	provide with a base, support, lining, or backing
	put (something) under or beneath
	raise or support (the level of printing) by inserting a piece of paper or cardboard under the type