Tech-ply
From BelegarthWiki
A specific brand name for 1/4in 5-ply Birch Plywood Underlayment that is sold in sheets of 4'x5' or 4'x8'. This is the most common core type for lightweight strap shield cores. Most lumber yards are not going to know what Tech-Ply is, so be prepared to refer to it as 1/4in 5-ply Birch Plywood Underlayment.