Hi,

We're remodeling and the old flooring (lino and carpeting) has been taken
up to install hardwood. I noticed many of the 4'x 8' x 3/16" underlayment
plywood has lines of stitching. Yes, stitching, which goes through to both
sides, and is made of nylon or cotton multi-strand thread.

What is this? I've never seen it before.

TIA