Rag is a papermaker’s term dating back to the 15th century when cotton rags and trimmings were the principal raw materials used for papermaking. The term continues to be used to describe papers and boards made of 100% cotton fiber pulp. Regular mat board is made from wood pulp which is buffered (pH balanced) during manufacture. The core and back of the board are acid-free, but not the surface paper. Rag mat board is made from cotton. The core, backing and surface paper are acid-free. Rag mat boards come in 2 thickness – 2 ply and 4 ply. 4 ply is roughly the same thickness as regular mat board.