GSM.

Grams per square metre: the mass of one square metre of the stock, and nothing beyond that. It is not thickness, not fibre, and not the coating that decides how ink sits on the surface. Epson publishes 395 gsm and 23 mil for its Exhibition Canvas Matte, two numbers in the same spec block, because weight and caliper move independently and a buyer who reads one has not read the other. The same split runs through transfer paper, where Beaver Paper offers one TexPrint grade at 92, 72 and 58 gsm: the number answers how much ink the sheet has to hold, not how good the sheet is. Shops still buy canvas on grammage alone, then find the heavy stock creases at the corner of a gallery wrap, or that a light paper cockles under a full ink load and takes a head strike with it. Ask for caliper, base fibre and coating; on its own, the figure has told you the shipping weight.