... free mat board 2,Various color and multi openings. 3,Professional design and quality. 4,Economic Price. 1. ... core, black core of mat board3. Lignin free and acid free4. Thickness: 0.5mm--3.0mm5. ... reverse bevel cut, V-groove 6. Multi layers and multi openings7. 4*6", 5*7", 8*10 ... ", 27*31" and other sizes are available8. A variety of design, prints and colors9. Professional quality and economic price.