Knotting of Oriental Rugs and Carpets
In the Bhadohi - Varanasi area of India, Oriental rugs are graded using two numbers, such as "5/40," "9/60," or "12/60." The first number is termed bis and it represents the horizontal (width) knot count. It is the knots in 9/10 of an inch of the rug's width. If you take 11% of the bis and add it to the bis the number you get is the knot per horizontal inch.
The second number is called "bhutan" and represents the knots in 4 1/2 inches of the rug's length. So if you multiply the bhutan figure by .333 and add it to the bhutan you get the number of knots per vertical inch.
Since bis is the knots in 9/10 of and inch and the bhutan is the number of knots in 4 1/2 inches, the following calculations can be made: 9/10 of 0.9" x 4.5" equals 4.05", almost four square inches, so an easy conversion is to multiply the two numbers together and divide by 4 (sq. in.) to get the approximate knots per sq. in. In a "9/60" quality rug you would make the following calculation; 9 X 60=540 and 540/4=135 knots per sq. in.
In Pakistan, quality names like "16/16" or "16/18" for rugs in Persian design represent the number of knots per linear inch across the warp and weft counted in the normal way. For example, a "16/18" quality Pakistani Tabriz has about 288 knots per square inch (16 x 18). In the trade, these are nominal counts and rarely are the actual knot counts the same as the nominal counts.
"Bokharas" from Pakistan in qualities like "9/18," "10/20," "11/22," and "12/24" are different. In this type of rug, warps lie in the same plane and are not offset, so both nodes of the knot show on the back of the rug. A 12/24 rug would have 288 knots per square inch.
China uses a completely different measure of knot count called "line counts" as in "80 line," "90 line," or "120 line." The line count equals the number of knots (actually pairs of warps) in a linear foot measured across the width of the rug. Thus a "90 line" rug has 90 knots per linear foot across its width. So 90 divided by 12 equals 7.5 knots per horizontal. In Chinese rug construction, there are an equal number of knots vertically so 7.5 x 7.5 equals about 56 knots per square inch; a "160 line" rug has about 177 knots per square inch and a "240 line" rug has about 400 knots per square inch.
Iran uses the term, "Raj" ("reg") which is the number of knots per "gereh." A gereh is 7cm. which is approximately 2 3/4 inches. Twenty four raj would be approximately 76 knots per square inch. Most rugs now coming from Iran are measured in knots per sq. centimeter.