Wall-to-wall carpeting is now sold in widths from 12 feet to 15 feet (“broadloom” simply means the rug is at least 12 feet wide).

Most of the numbers given to describe carpet's construction concern how densely yarn has been packed into the pile, which, other things being equal, determines service life.

is the number of warp lines of yarn in a 27-inch width. Twenty-seven inches was a standard width for woven carpeting in the 19th century. The greater the pitch the better.
Stitch rate
which applies to tufted carpet, is the number of yarn tufts in one lengthwise inch. Sharply bending the carpet reveals the tufts for counting. Seven to eight tufts per inch is pretty good; three or four is poor. The distance between the centers of tufts is called the gauge.
Pile height
is measured from the surface of the backing to the top of the pile. High pile does not wear as well as low pile, but a high-density high pile wears better than a low-density one, because the fibers support one another. Modern heat-set twists, used on synthetic yarns, are also helpful.

The amount of yarn in a carpet can be summed up by its weight, usually given as ounces per square yard. The total weight includes everything, backing and all. Since the backing may include heavy coatings that might not influence the wearability of the carpet, a much better measure of quality is face weight. Unfortunately, many stores and manufacturers now refuse to reveal the face weight. A face weight of 40 to 50 ounces is a serviceable carpet, although lower face weights might be appropriate in low-traffic areas such as bedrooms. Below 30 ounces, beware.

Area rugs

A figure of merit for oriental rugs is the number of knots per square inch. An ordinary rug may have 50 knots per square inch, and a fine rug 250 or so. Rugs with as many as 600 knots per square inch are known; though they are beautiful objects, it is hard to admire them without recalling that tying such rugs destroyed the eyesight of many children.

The knot density of modern Chinese rugs is sometimes described in lines. A 90-line rug has 8,100 knots per square foot, the “90” being the square root of the number of knots in a square foot. Dividing by 144 square inches in a square foot shows that 90-line is equivalent to 56.25 knots per square inch.

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