(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
The first step in discussing any paper board is to define a board. A paper board is defined by its thickness. Any paper that is 0.012 inch or more, which is a little less than 1/64 inch in thickness, can accurately be called a board. Tissue paper is approximately 0.001 inch thick, bond paper ranges from 0.003 to 0.004 inch, and one-ply museum board ranges from 0.0125 to 0.015 inch. The material most commonly used to store and protect fine art is four-ply museum board and its thickness ranges from 0.050 to 0.060 inch (0.060 inch is almost 1/16 inch.
At one time there were several ways to define various boards by their thickness. A standard mat board was said to be fourteen-ply (roughly the equivalent of putting together fourteen sheets of bond paper) and double-weight mat board twenty-eight-ply. A standard museum board, which is approximately equal to a mat board in thickness, is called four-ply because each ply is much thicker than bond paper and it takes only four to reach the desired thickness. Today, most manufacturers of paper boards are shifting toward using a point system of measurement, which will make it easier to comprehend thickness and to compare various boards. In the point system, one point equals 0.001 inch. Therefore, if the average mat board or museum board is between 0.050 and 0.060 inch it would be between 50 pt. and 60 pt. in thickness.
There can often be as much as a four-point variance of thickness within a manufacturer's advertised thickness. Manufacturers of mat board have, for the most part, dropped their system of defining thickness by plys in favor of the point system. Some manufacturers who make both mat board and boards for illustration, as well as for graphic arts, refer to their 50 pt. to 60 pt. boards as single weight, and their 100 pt. to 120 pt. boards as double weight. Museum boards are still most commonly referred to in plys, although the point system is rapidly taking over.