18mm thick Baltic birch ply wooden panel with a slot cut in the surface to form a 'frame'.
The size refers to the painting surface size ie a 300mm panel has a painting surface of 300mm diameter with an additional 6mm slot and 12mm wide frame.
The slot is 10mm deep, 6mm wide.
Each panel is therefore 36mm larger in diameter than the stated size.
A key hole is routed into the reverse for easy hanging.