The shortest pile tier in one dealer's four-tier scheme, about 1/8 to 1/4 in (3.2 to 6.4 mm) measured from the back of the rug to the top of the pile; the next tier, low-medium, runs to 1/3 in (8.5 mm). It feels smooth and shows a sharp, high-resolution design, so finely knotted rugs, silk Tabriz and other pure silk rugs are trimmed low, as are antique rugs worn down by age and antique-washed Oushak and Kazak rugs. It clears doors that swing over it, absorbs less humidity and dust and vacuums more easily, but bears heavy traffic and furniture less well than a taller pile.
A rug whose pile is entirely silk, woven on a cotton foundation or on a silk foundation; when both are silk, no wool enters the rug. Silk's fineness allows the highest knot density of any Persian rug, roughly 600 to 1,000 knots per square inch (about 9,300 to 15,500 per 10 x 10 cm), so the pile is kept low to show the design, and the fibers give a luminous sheen that shifts with the light. About 90 percent of Qum rugs are silk on both foundation and pile, as are some Isfahan rugs. These are the most expensive Persian rugs; they are delicate and stains are hard to remove, so they suit display, formal rooms or wall hanging rather than dining rooms or homes with pets.