In some carpet styles the looper then rocks back against a knife, where the small loops of yarn are cut, creating what we call a cut pile carpet. The length of these cut pieces of yarn is referred to as the pile height and is basically the distance between the looper and the primary backing.
This method of selectively cutting, called cut and loop construction, creates a recognizable pattern on the surface of the carpet.
Pile height, or nap, is the length of the tuft measured from the primary backing to the yarn tips. It is usually shown as a fraction, or sometimes its decimal equivalent. Usually shorter pile heights are more durable than longer pile heights.
The stitch rate of a carpet is the measure of how close the yarns are together. Stitch rate is measured in penetrations, or tufts, in each length of carpet, usually an inch. The stitch rate is controlled by how fast the carpet is moved through the tufting machine. Seven to eight tufts per inch is a good number, while three or four is poor.