Reusable - Simply flip over or turn upside down! Easy to label with a permanent marker or affixing a label. 30mil Thick 12" x 13-1/8" - Slanted on One Side They're durable, and feature rounded corners to prevent jabbing yourself! Our divider cards are professional heavy duty store grade divider cards. They are made of 100% virgin High Impact Polystyrene (H.I.P.). They are strong stuff - it's what we sell to music stores all over the country! These cards are slanted on one side with rounded corners (it eliminates jabbing yourself on the sharp corners!) We believe these to be superior to the "tab" style dividers because those are more likely to snap off These dividers are reusable - if you don't need that section anymore, just turn it around!