I get calls every week about the thickness of different matboards and like so many other topics that have to do with matboards there is a lot of misinformation out there. I can only describe the paper industry in North America as totally confusing while the Europeans make it a little easier. Europe uses the grams per square meter method of measuring the basis weight of paper which does not rely on anything but a square meter of any paper. Here in good old North America we use pounds per ream. Now a ream is 500 sheets of paper but the paper sizes change depending on the type of paper you are looking at. A ream of text [matboard surface paper] is measured at 500 sheets of 25 x 38 inch sheets or about 3300 sq ft. Cover paper is measured at 20x26 inches times 500 sheets. Bristol,Bond,,Index, and Tag are all different sizes again. Basis weight has pretty much nothing to do with thickness only weight per 500 sheets. Thickness is also called caliper and as basis weight goes up usually caliper follows but most times weight is more important than caliper as paper is sold by the pound. Are you confused yet. it gets better. A ply is 15 points or 15 one thousandths of an inch. A 4 ply matboard [100% cotton Museum] is 4 x 15 or 60 points. This only really refers to museum boards in framing and really for no other reason than it is tradition. For the most part, Conservaton, Whitecore, and Creamcore are not measured in plys by manufacturers. They are nomally 50 to 55 points depending on who made them, how much they will sell for, and what they will be used for. That is just the way our industry has gone. We call some of our boards double thick. This means we put two cores together before laminating the surface and backing paper. This makes the caliper about 95 points which is less than a true 8 ply [120 point] museum board.