For Isfahan rugs, the number of knots per square inch (KPSI) is measured by the "Kheft" rather than the concept of "raj" in other types of Persian rugs such as Tabriz. If you look closely at the kilim border right before the fringe starts, you will notice a line of either red or blue at regular intervals. This is a mark showing that there are 100 threads of warp in that interval. So an Isfahan rug in any given size could have lower or higher KPSI. If you count 14 intervals separated by such colored lines in a certain piece (call it A) across the rug, it means there are 1400 warp threads on which the knots have been tied. Now if you get a piece of the same size (call it B) and you count 16 intervals, you have 1600 warp threads, thus a higher KPSI, and most likely, a more expensive piece. Rug "A" is considered a higher quality Isfahan compared to rug "A". Each one of these intervals is called a "Kheft". Of course, there are many more factors to be considered in evaluating an Isfahan rug, but "Kheft" is an important one. I hope it answers your question.