Not a ruler. A straightedge is ground flat along its working edge to a tolerance of about 0.05 mm over its length, and this one is 3 mm stainless, 300 mm long, 32 mm wide, with one edge bevelled to a 30-degree ruling lip.
The bevel is why it exists. A flat-edged ruler sits in contact with the paper and a wet nib drawn along it wicks under the edge and leaves a blot on the underside. A bevelled lip holds the metal off the sheet by half a millimetre so ink cannot reach it. Anyone who has ruled a line in fountain pen with a plastic ruler knows this problem.
The back is faced with 1.5 mm cork, bonded across the full area, which stops it sliding and raises it further still. Graduated in millimetres along the unbevelled edge, etched and filled in black. The mark is etched at the 300 end.
