A Persian grading scale specific to Nain rugs, in which a number such as 4, 6 or 9 counts the layers (la, "layers") of thread at each warp position. The scale runs backward: the lower the number, the denser the knotting, typically about 650 KPSI for 4 la, 400 to 500 for 6 la and 200 to 300 for 9 la (roughly 10,100, 6,200 to 7,750 and 3,100 to 4,650 knots per 10 by 10 cm).
A rug whose pile combines silk and wool, usually mostly wool with silk yarn knotted into chosen details such as floral motifs to add sheen and sharpen the pattern. Silk is finer than wool and permits denser knotting, roughly 400 to 700 knots per square inch (about 6,200 to 10,850 per 10 x 10 cm) on a cotton foundation and 500 to 1,000 (about 7,750 to 15,500) on a silk foundation, although Nain, almost always on cotton, drops lower in its coarser la grades, where a higher la number means fewer knots. The wool keeps the pile thick and durable and the price below that of pure silk, though above that of wool alone. Tabriz, Nain, Mashad, Bijar, Isfahan and Qum all weave these rugs, usually with the pile cut short to show the detail.