Specially trained rug weavers tie each knot by hand to build a rug design when making hand-knotted rugs.  Hand-knotted rugs require greater skill and a much more labor-intensive construction process than all other rug construction methods.  The surface of hand-knotted rugs is created by tying individual knots using certain knot styles. Knots are visible on the back of the rug, and they mirror the pattern on the front. The number of knots per square inch shows the quality of a hand-knotted rug. Every hand-knotted rug is an art-piece and each one is unique.  No two are alike. These rugs can retain/gain value through time.  They are mostly made of wool, yet there are also a large number of silk or silk and wool combination rugs.