How to Count Rug Knots....
Knot density (knots per square inch) is an important indicator of rug
quality. Most weaves are measured simply by counting the number of knots per
linear inch along the warp (i.e., along the length of the rug) and the number of
knots per linear inch along the weft (across the width of the rug) and
multiplying to get the number of knots per square inch (or per sq. cm.).
Unfortunately, this simple concept can be tricky to apply in practice.
Because of the ways in which rug structure can vary, individual knots can be
difficult to isolate from the back of the rug (it's impossible to distinguish
separate knots from the face of the rug).
 This is one Turkish knot, even though the wool wraps around two warps. 
Often the warps of the rug lie on the same plane. If the warps of the rug lie on the same plane, each knot (whether Turkish or Persian) will show on the back of the rug as two tiny squares of the same color next to each other across the width of the rug.
 The warps of this Turkishknotted rug lie on the same plane. 
 You are looking at
one creamcolored Turkish knot (surrounded by navy knots and red
wefts) from the back of a Turkish rug. Can you see the two
sidebyside elements of this knot? If you are counting the knots
in this rug, the two cream bumps count as one knot.

Sometimes the warps are offset so greatly that from the back of the rug alternate warps are hidden. If this occurs, each knot (whether Turkish or Persian) will show on the back of the rug as a single tiny square of color.
 The warps of this Turkishknotted rug are offset. 
You are looking at one light blue Persian knot (surrounded by pink knots) from the back of a Pakistani rug. Because alternate warps are so strongly offset, you can only see one element of the knot across the width of the rug. If you are counting the knots in this rug, the one light blue bump counts as one knot. 
How do you know when to count one bump on the back of the rug as one knot? It's easy  look carefully at the individual areas of color across the width of the back of the rug. If you only see colored elements in pairs, you need to count each pair as one knot. If you see lots of single colored elements, the rug has offset warps and each element should be counted as one knot. Many country rugs from Turkey, Afghanistan, and Iran show both knot elements on the back of the rug, as do Bokharas from Pakistan. Most rugs from India and China have strongly offset warps, and so show only one knot element on the back of the rug.
Think you've got it? Take our knotcounting quiz.
In the Varanasi area of India, rugs are graded using two numbers, like "5/40," "9/60," or "12/60." The first number represents the knots in 9/10 of an inch of the rug's width. The second number represents the knots in 4 1/2 inches of the rug's length. 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 weave in knots per sq. in. For example, with a "9/60" quality rug, 9 X 60=540 and 540/4=135 knots per sq. in. Note that most rugs from India have strongly offset warps, so you will only see one element of the knot on the back of the rug.
In Pakistan, indicated qualities 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 Kashan has 16 X 18 weave, or about 288 knots per sq. in. Note that these rugs have strongly offset warps, so you will only see one element of the knot on the back of the rug.
Socalled "double" Bokharas from Pakistan in qualities like "9/18," "10/20," "11/22," and "12/24" are different. In this type of rug, warps lie sidebyside and are not offset, so both elements of the knot show on the back of the rug. Be sure not to doublecount the weave across the width when examining a Bokhara.
China uses a completely different nomenclature, with "line counts" like "70 line," "90 line," or "120 line." The line count equals the number of knots 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. A "90 line" Chinese rug has about 56 knots per sq. in.; a "SINOPERSIAN" (a Chinese rug in Persian design) in "160 line" quality has about 177 knots per sq. in.; a "240 line" rug has about 400 knots per sq. in. Chinese rugs have strongly offset warps, so you will only see one element of the knot on the back of the rug.