Flat weave rugs have been made for millennia. Extremely versatile and popular, flat weave rugs have no pile and are easily rolled and transported. Traditionally used for multiple purposes including rugs, bed covers, curtains, and canopies, popular flat weaves like Indian dhurries and tribal kilims can be plain weave (warp and weft are visible) or "slit-tapestry" weave where the warp is hidden ("weft faced") – the same technique used to make European Aubusson carpets. Flat weave rugs are faster to weave than hand-knotted rugs. Tribal flat weaves are traditionally woven by nomads or semi-nomads (see Tribal & Geometric). Other flat weave rugs include Senneh kilims woven in the western Iran where they also made pile rugs and soumak flat weaves. Soumak carpets are stronger than kilims because weft yarn fibers are wrapped over two warps and under one (or over four and under two) which securely anchors the fibers to create greater durability. Kilims are often reversible, but some have warp "floats" or tag ends on the reverse; time-saving techniques that create a one-sided kilim. Traditional soumak weavings have a "braided" face and a "ragged" reverse because the tag ends of yarn are left to hang on the back. New Soumak rugs have a "clean" back. Kilims can have a continuous single color weft or dual color continuous weft where the colors are woven as a pair ("complementary"). Most kilims are patterned which means they have a "discontinuous weft". Slit-tapestry is named after the small opening that occurs when colors are changed ("discontinued") on a vertical area of the design. When a color changes on the vertical such as the left side of a square, the weft fiber is doubled back and "discontinued" on the warp, and a new color is continued on the adjacent warp. This creates a slit or a gap. "Dovetailing" describes the effect created when two different weft colors meet from opposite directions and are interlocked on the same warp (there is also a "double interlock" technique). Supplementary weft can be added to the existing weft in a technique called "brocading". Kilim slits are features, not flaws, whereas gaps on European slit tapestry carpets are sewn shut after weaving. Slit-tapestry technique produces sharp pattern delineation and a smooth weave whereas dovetailing creates less crisp patterns.