The warp threads (vertical) are stretched between two beams and the weavers, sitting side by side every 90 cm, and patiently giving birth to the rug, knot after knot, row after row, grasping the appropriate coloured strand at each knot. Weft threads (horizontal) are interwoven in the warp threads between each row of knots. This is how the rug backing is built up and consolidated. The warp and weft threads are usually made of cotton, but can be made of wool to produce nomadic rugs and in silk for fine rugs The vertical loom is also referred to as the high wrap loom.
To make each knot, the weaver selects a thread of the colour corresponding to the design and ties a knot around the 2 weft threads.
A knot tightening of 5/5 is equivalent to 25 knots per square inch. By multiplying this number by 1,550 we get the number of knots per square inch : 38,750. A knot tightening of 8/8 is therefore equivalent to 99,200 knots per m² and a 10/10 to 155.000 knots per m². There is an other classification based on non-metric units of measurement. For instance, a tightening of 7/18 is equvalent to 48,222 knots per m². To make a simple calculation, multiply the first and second numbers and then the result by 383. A tightening of 9/45 is therefore equivalent to 155,115 knots per m².
In a rug with a density of 100.000 knots per m², the head of each knot can be seen on the pile. This gives the carpet an interesting “material effect”. However, if after weaving the rug is washed twice or three times, the wool strands will open up and become “pile-like”.