In general, quality oriental rugs command a higher price than coarsely knotted ones, if only because the material cost is higher and they take longer to produce. Nevertheless, the knot density is by no means the sole criterion of quality. There is as yet no fully agreed terminology for knot density although in recent years the following classification has come into use:
|up to 500 knots per sq. dm (30 per sq. In.)||= very coarse|
|500-1,000 knots per sq. dm (30-60 per sq. in.)||= coarse|
|1,000-2,000 knots per sq. dm (60-130 per sq. in.)||= medium fine|
|2,000-2,500 knots per sq. dm (130-160 per sq. in.)||= fine|
|2,500-4,500 knots per sq. dm (160-290 per sq. in.)||= very fine|
|over 4,500 knots per sq. dm (290 per sq. in.)||= extremely fine|
Knot densities of over 4,500 knots per sq. dm. (290 per sq. in.) are rare in antique rugs, although they are not unusual in quality rugs with silk pile. A few well known wool carpets have knot densities over 10,000 knots per sq. dm. (650 per sq. in.), e.g. the Vienna Hunting Carpet has 12,700 knots per sq. din. (1,000 per sq. in.) while the finest silk rug known has a knot density of about 40,000 knots per sq. dm. (3,200 per sq. in.).
In the carpet trade knot densities are sometimes given per meter, Le. the number of knots per 10 cm is measured and multiplied by ten. The more accurate method of determining the number of knots per square decimeter or square meter is however to be recommended. Moreover, useful information is sometimes contained in the individual horizontal and vertical knot counts. The number of knots should be counted in a full 10 cm; multiplying up from knots per cm is unsatisfactory. Since the weave of a carpet is rarely uniform the most accurate results are obtained by determining the knot density in two or more parts of the carpet. When counting knots in the horizontal (weft) direction it is important to remember that each knot passes over two warps.