Persian rugs are categorized into types named after the Persian city, village or tribal area where they were woven. The most renowned cities for producing rugs are Isfahan, Kerman, Kashan, Herat, Nain, Tabriz, which generally wove wool rugs, but the city of Qum is renowned for silk rugs. Weaving centers are sometimes located near grazing land for sheep who produce the wool for hand spinning. While red ground rugs are commonly seen as "typically" Persian, colors vary greatly and include pastels. Persian rugs are a sub-group of a category of rugs known as "Oriental". The majority of rugs sold today as "Oriental" are woven with Persian designs, but while all Persian rugs are Oriental rugs, not all Oriental rugs are Persian rugs. Antique rugs (older than 100 years) are described as Persian if they were made in historical Persia. Persian rugs usually have a wool weft on a cotton warp that becomes invisible during the hand weaving and knotting process. Tribal rugs were typically made suThere are different knotting techniques; for example, the Persian or asymmetrical knot (open on one side) is tied (knotted) to a warp, passes over the next warp, ties to the next warp and so on. Each knot is cut to create the pile, whereas other weavers use a long rod inserted between warps to allow an entire row of pile to be sheared at once. Unlike tribal rugs for which patterns were memorized, the intricate designs of Persian rugs are graphed onto paper and displayed behind the upright loom as a guide for the weavers. European and North American demand for "Persian" rugs took off in the 19th century. Trading companies sprang up to satisfy demand, but western taste didn't always match the export rugs. Adapted designs were woven which diluted their origins thus making the broad term "Persian rug" more common. Many Persian designs feature plants, gardens, and animals, often interlaced or bordered with geometric patterns or featuring central medallions.