I looked up over-lay and under-lay on Google, and what came up were sewing patterns, where there were pieces that overlapped, one being the overlay and one the underlay. I don't know how much this helps, though.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry