Q. What is the difference in 2 ply, 4 ply, 6 ply and 8 ply? Which ply do I need?

A. The term 'ply' refers to the thickness of the mat board. The higher the number, the thicker the board. The most common thickness used is 4 ply (around 1/16th of an inch). You can tell the difference in thickness by saying 2 ply is 1/2 the thickness (1/32") of 4 ply. 6 ply (1/8") is 4+2 ply, so it would be 1 1/2 times as thick. 8 ply (1/4") would be 4+4, or 2 times as thick. It is double the size of 4 ply (double the thickness). Which ply should you use? That depends on what you are doing! 98% of all mat board sold is 4 ply. 6 and 8 ply mat is often used in museums or galleries for special presentations of artwork or photos. The 45 degree bevel allows the extra thick core to show and gives a dramatic effect. Double matted means you will have 8 ply mat on your artwork if you use the 4 ply board. In most cases, 4 ply works great, and anything more is up to you! If you choose a thicker or thinner mat board, you must adjust the blade on your mat cutter to avoid over or under cuts. If you are just starting out in mat cutting use 4 ply only. Also, some mat cutters (both manual and computerized) will not cut 8 ply. If you're interested in a great machine that can handle an 8 ply board, check out the Logan 850 mat cutter!

from: http://www.framingsupplies.com/MatBo...atMatBoard.htm