An absorbent material made from plant pulp or fibrous substances, usually in thin sheets or leaves, dyed or bleached to a desired hue, pressed or polished into a desired texture, and variously sized for writing or printing. Paper quality is determined by content (eg: cotton rag, wood pulp, or recycled paper), with acid-free archival stock being the most enduring. The standard estimate for calculating the average thickness of paper is: 100 pages = 1 centimeter. Paper thickness is a factor of the basis weight on a ream of identical pages; but dissimilar types of paper (eg: bond vs newsprint) are not easily compared. The word 'paper' derives from "papyrus", which was formerly used as a writing surface; despite the papermaking process having been invented in First Century China. Types of paper include: art paper, bible paper, bank, board paper, body stock, bond paper, dual-purpose bond paper, book paper, Bristol board, Bristol paper, cardboard, cartridge, catalogue paper, chipboard, corrugated board, cover paper, demy, duplex paper, equivalent paper, fine papers, flimsy, foolscap, form bond, free sheet, glassine, groundwood paper, house sheet, ivory board, job lot paper, kraft paper, laser bond, ledger paper, legal paper, letter paper, lightweight paper, manila, manifold, monarch, NCR paper, onionskin, opaque paper, parchment, pasteboard, publishing paper, recycled paper, royal, strawboard, supercalendered paper, vellum. Paper sizes include: A sizes, A4 paper, B sizes, basic size, caliper, C sizes, cut sizes, ISO sizes, JIS sizes, legal-size, letter-size, parent sheet, P sizes. See sheet, quire, ream, basis weight, substance weight, grammage, CWT, hundredweight, M weight, ruleup, recto, verso, backtrack, opacity, grain direction, grain long / short paper, cross grain, with the grain, felt side, wire side, grade, condition, pulp, swatchbook, off-shore sheet, mill order, making order, make-ready, carton, carload, pre-consumer waste, broke, post-consumer waste, waste, spoilage.