Underlays

noun

1. A layer (of earth, etc.) that lies under another; substratum.
2. A soft floor covering that lies under a carpet.
3. Anything that is underlaid.
4. A piece of paper pasted under woodcuts, stereotype plates, etc. in a form, to bring them up to the necessary level for printing.
5. Lyrics; or more specifically, the way in which lyrics are assigned to musical notes.

   "The underlay in bar 3 is unclear in Handel's manuscript."