No. 99 is a single thick (14 ply) media board that is particularly good for markers. It has a medium textured surface and a gray core. No. 300 is also a single thick board (16 ply) but is more recommended for various artist media such as soft pastel, charcoal, acrylic, watercolor and gouache. It too has a medium textured surface and a gray core. No. 300 has a high rag content sheet made for professional needs. No. 310 is a fantastic single thick (16 ply) artist board. It is highly recommended for oil pastel, soft pastel and charcoal painting or rendering. It features a 100% rag, acid-free surface and gray core.