There are three different methods of identifying the knot count in Persian Carpets.
The most widely used is KPSI, short for knots per square inch, where the number of knots along the horizontal axis are times by those along the vertical in a measured grid of one square inch.
Alternatively, "Raj" is sometimes used, particularly in rugs from Tabriz and Bijar. Raj denotes the number of rows in a "Gereh" which is 7 cm of weft threads counted vertically. A fine Tabriz rug may have a count of 55 Raj, and a coarser one around 25 Raj and would generally be priced according to these guidelines as well as the general execution of the piece.
Nain rugs are measured differently, by "Laa" which is the term given to the cotton threads in the warp; Nola is a 9 ply warp technically 3 three ply strands corded and is generally among the coarser of Nain output. Shisla is a 6 ply warp technically 3 two ply strands corded, typically dense in structure and finely detailed. Charla is a 4 ply warp technically 2 two ply strands corded, extremely fine, with substantial highlights of silk and seldom produced.
Overall these guidelines are telling of fineness but there are many more factors involved in determining the value of a Persian carpet; such as colour combination, wool quality, age, straightness, clarity and also if the pattern is unusual or the piece was created by a specific Masterweaver, these are all relevant and point to the importance of research and utilizing reliable dealers as a source in selecting a carpet for your home, to which - I would probably stay away from the "Bankruptcy prevention" and "80% off for today only, and maybe tomorrow also" sales of this world.
