Made of complex, twisting, ancient melodic patterns (called ragas or raags) and studied, hypnotic rhythms (talas), Indian classical music is divided into the Islamic-influenced Hindustani tradition of the north and the Carnatic of the south. Performers of both styles spend countless hours learning how to establish hundreds of different moods, developing astounding technique and assimilating intricate rules that have been passed on for thousands of years. Most Indian instrumental music aims to emulate the human voice using a wide variety of string, wind and reed instruments. Musicians in the north favor the lute-like sound of the sitar, while those in the south depend on the similar, if somewhat heavier, tone of the veena. A variety of skin-drum percussion instruments, such as tabla in the north and mrdingam in the south, provide rhythmic accompaniment. The droning sounds produced by the four-string tambura give Indian music its trademark haze. From the slow, ponderous ascent of a primary theme in the opening section, to a blisteringly fast sitar improvisation, Indian classical music challenges listeners' intellect and emotions.