"underlay" in the noun sense
1. carpet pad, rug pad, underlay, underlayment
a pad placed under a carpet
"underlay" in the verb sense
1. underlie
be or form the base for
"underlay the plate"
2. underlie
lie underneath
"They underlaid the shingles with roofing paper"
3. underlay
provide with a base, support, lining, or backing
"underlay the boards with joists"
Source: WordNet® (An amazing lexical database of English)
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