Most Sherpas live in the eastern regions of Nepal Solu, Khumbu or Pharak. However, some live farther west in the Rolwaling valley and in the Helambu region north of Kathmandu. Tangboche is the Sherpas' oldest village in Nepal, and is estimated to have been built over 300 years ago. Sherpas speak their own Sherpa language which in many ways resembles a dialect of Tibetan. They traditionally are traders and farmers, cultivating their high altitude fields of potatoes, barley, wheat and buckwheat. Some live near Namche Bazaar. In India, Sherpas also inhabit the towns of Darjeeling and Kalimpong and the Indian state of Sikkim. Sherpa society uses a clan system or ru to determine marriage arrangements. The clan system is believed to have originated in Kham, where ru (literally, 'bone') clan & marriage system is still in use today. Clan identity is inherited through the father. Traditionally, each Sherpa would select a spouse from a clan outside their own. For example, a Lama cannot marry another Lama, but a Lama is free to marry a member of any of the other seventeen clans.