What is "knot count" and how important is it?
Although knot count is an important factor and is good to take into consideration, it is not the end all be all when determining the quality of a hand knotted rug. There's a lot more than meets the eye.
Knot count, also known as knot density, is the number of hand tied knots within a certain area of a rug. Determining knot count must be done by close examination of the back of a rug. Understanding knot count requires some understanding of how rugs are constructed. Hand knotted rugs are made up essentially of three elements, the warp (A - the vertical fibers), the weft (B - the horizontal fibers) and the knots (C - which create the pile).
Counting Rug Knots
The warps of the rug usually lie in the same plane, and when this is the case, each knot will appear on the back of the rug as two squares (nodes) of the same color next to one another from east to west on the rug. Sometimes the warps lie offset, this causes the second node of the knot to be hidden from the back. When both nodes are visible, they are counted as one knot. It is a common mistake to count both nodes as two separate knots.
The two most common knot types used in rug making are the Persian knot and the Turkish knot. The offsetting in warps can occur with both Persian knots and Turkish knots. See the illustration below to compare the structure of both knot types. You can easily see how if the warps are lying in the same plane the knot will have two nodes. (Persian on left, Turkish on right)
A good general rule to follow when counting knots is: If you only see colored elements in pairs, you need to count each pair as one knot. If you see many single colored elements, the rug has offset warps and each element should be counted as one knot.
Now that we know what we are counting, the knot count is the actual number of knots in a set area, typically that area is one square inch. To determine knots per square inch (KPSI), count the number of knots within a one inch vertical and one inch horizontal area then multiply the two numbers. A rug with 10 knots horizontally and 10 knots vertically will have 100 KPSI.
What's in a Number?
It is a common misconception that if the knot count increases, the quality of a rug increases. In actuality, good quality Persian rugs can range in knot count from 85 KPSI (Like this Heriz) to over 500 KPSI (Like this Qum). It can be assumed as a general rule, that as the knot count increases, the cost of the rug will increase - especially when buying a new rug. The more knots, the longer it takes to make. For example compare a 50 KPSI and a 100 KPSI rug, there are two times the number of knots per square inch in the 100 KPSI rug, that means it will take twice the amount of time to create the same one square inch of rug, this increase in time equates to a higher price.
Finest knotting (440 KPSI) on the left to the chunkiest knotting (20 KPSI) on the right.
Knot count does tell us the fineness the rug, this is important in that it tells us the level of detail a rug will have. The finer the rug the more curvilinear the design can be. Like pixels in a photograph the higher the count, the smoother and less pixellated the image becomes. Whether the pattern is a fine, curvilinear floral (like the rug on the left below) with a higher knot count, or a less fine, geometric pattern (like the rug on the right below) with a lower knot count, just knot count alone does not imply that one rug is better quality than the other. Both rugs are attractive, good quality examples of their types.
Knot count can often tell us a lot about where a rug was made. Typically rugs with a higher knot count are made in city environments, where weavers are sedentary and the rugs are being made to be sold. Less fine rugs were often made by nomadic tribal groups and were being made for their own use (this isn't always the case with newer rugs). Experts use knot count as one of the factors (along with many others) to help them determine the origin of an antique rug.
Knot count is not always a direct indicator of cost/value. True antique Persian rugs are valued for their age, condition, and appeal, in addition to their knot count. As with most things, looking at just one element instead of the whole does not give you an accurate sense of quality and value. It is the coalescence of multiple aspects that attest to the quality and value; including age, colors, condition, and general appeal of the rug. In other words don't make any rug buying decisions based on knot count alone. There are many other aspects of a rug that should be taken into consideration. As we always say, buy a rug because you love it not because it has a particular knot count.