Knot density, otherwise known as knots per square inch, are an important indicator of rug quality. Most knots are measured on the reverse side of a hand knotted oriental rug. Simply counting the number of knots per linear inch along the length of the rug and the number of knots per linear inch along the weft, and multiplying these two number, give you the of knots per square inch.
Raj are the units of knot density used primarily in Iran for Tabriz rugs. Raj refers to the number of knots across 2 ¾ inches of a rug.
Knot density is not the most important factor in purchasing an oriental rug. Knot count can vary greatly depending on where the rug was made, how it was made, and the tradition behind the construction.