On Mon, 13 Jul 2015 16:44:40 -0700 (PDT), TimR
wrote:

On Monday, July 13, 2015 at 6:12:47 PM UTC-4, Boris wrote:
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

Very common in boatbuilding to stitch plywood then epoxy over, but I've not seen it on a floor.
I've never seen plywood underlayment stitched, but "fingerjointed
hardwood "or "fingerjointed parquet" is often stitched with nylon
cord.