approaches the maximum a skilful weaver can tie per hour. In the late fifteenth century a "carpet design revolution" occurred, made possible by finer yarns, and before this time it is rare to find carpets with ≥120 kpsi but by the next century carpets with three to four times that density were fairly common. For example, the Pazyryk carpet (ca. 400 BC) is around 234 kpsi and the Ardabil Carpets (ca. 1550 AD) are 300–350 kpsi. A fragment of a silk Mughal carpet in the Metropolitan Museum of Art has a knot density of 2,516 kpsi and a silk Hereke prayer rug (ca. 1970 AD) contains 4,360 symmetric kpsi. However, the rug with the highest knot density is a silk Hereke masterpiece by the Özipeks workshops, having an incredible density of approximately 10,000 kpsi, with a production time of about 15 years. (FN5a - https://new.liveauctioneers.com/item/53198792_probably-the-finest-rug-in-the-world-silk-hereke) In Persian, reg (raj, rag, Persian: "row, course") refers to the knots per gereh (Persian: "knot"), which refers to a unit of approximately . Dihari is a unit of 6,000 knots used to measure production in India. See also Knot (wood) Thread count References Rugs and carpets Density