Persian carpets are unquestionably one of the supreme Oriental art forms. The traditional techniques of Oriental carpet weaving have changed surprisingly little over several Millennia. The two principal forms of 'knotting' have been in existence for at least 3,500 years. Every carpet starts with a loom, a simple frame which gives the base threads the necessary tension to allow knots to be tied on them. The most basic form is the horizontal loom used by nomadic tribal weavers. Production of the more sophisticated village and city rugs varies only in the type of loom used, the quality of the yarns, and the degree of technical skill of the weaver. Carpets may be made solely from wool or from combinations of wool, cotton and silk. Some primitive tribal carpets incorporate goat or camel hair for greater strength and economy.