Knots per sq cm
Encyclopedia
Knot density is a traditional measure for quality of handmade carpet
Carpet
A carpet is a textile floor covering consisting of an upper layer of "pile" attached to a backing. The pile is generally either made from wool or a manmade fibre such as polypropylene,nylon or polyester and usually consists of twisted tufts which are often heat-treated to maintain their...

s. It refers, quite simply, to the number of knot
Knot
A knot is a method of fastening or securing linear material such as rope by tying or interweaving. It may consist of a length of one or several segments of rope, string, webbing, twine, strap, or even chain interwoven such that the line can bind to itself or to some other object—the "load"...

s per unit of surface area - typically either per square inch
Square inch
A square inch is a unit of area, equal to the area of a square with sides of one inch. The following symbols are used to denote square inches:*square in*sq inches, sq inch, sq in*inches/-2, inch/-2, in/-2...

 or per square centimeter.

For two carpets of the same age, origin, condition and design, the one with the higher number of knots will be the more valuable. Knot density is normally measured in knots per square inch
Square inch
A square inch is a unit of area, equal to the area of a square with sides of one inch. The following symbols are used to denote square inches:*square in*sq inches, sq inch, sq in*inches/-2, inch/-2, in/-2...

 (KPSI) which is simply the number of vertical knots across one inch of carpet multiplied by the number of horizontal knots in the same area. Average knot density varies between region and design. A rug could have a knot density half that of another yet still be more valuable, KPSI is only one measurement of quality and value in Persian Carpets.

Hand-tying of knots is a very labour-intensive task. An average weaver can tie almost 10,000 knots per day. More difficult patterns with an above-average knot density can only be woven by a skilful weaver, thus increasing the production costs even more.
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