In addition to the excellent responses previously, ply is also a function of how much a board is calendared. A calender is a device with two or more rollers through which the paper is run. The compression of the rollers and the application of heat give the paper the degree of its smooth and glossy properties.
Strathmore Bristol Board is the smoothest board that is made. It is calendared so much that 4-ply Bristol Board is thinner than 2-ply mat board. We use this Bristol Board in the dry mount press between the print and the platen. It is very expensive, but will last for thousands of dry mounting operations. We use two boards between the print and the platen and one board underneath. We cut the Bristol Board the same size as our mounting board.
Michael A. Smith