Read the dictionary definition of underlay. All definitions for this word.
1. a pad placed under a carpet
2. provide with a base, support, lining, or backing
1. underlay the boards with joists
3. put (something) under or beneath
1. They underlaid the shingles with roofing paper
4. raise or support (the level of printing) by inserting a piece of paper or cardboard under the type
1. underlay the plate