Laos is a club-shaped country with a squarish top and a long southeastward handle. All but the eastern bump of the club's head--Houaphan Province--is drained by the Mekong River system. That river--the world's tenth longest--comes from China's Yunnan Province above Laos's club head. It flows southwest to form the border with Myanmar(2), then turns southeast as part of the Thai border. There it is joined by the Tha, which originates to the northeast near the Yunnan border, in the indent in the top of the club's head. For a while the Mekong is within Laos, heading east and then south parallel to the Thai border, In this stretch the Ou joins the river, flowing from the nation's northernmost point. Again the Mekong becomes a boundary-marker with Thailand, this time for most of the remainder of Laos's west side. While still within the club head the Ngum adds its water, draining from the Plateau of Xianghoang in the club head's center and through the reservoir that is the nation's only lake, Along the handle the Bangfai and the Banghiang are among the Mekong's tributaries flowing from the Annam Mountains(3) that form the Lao-Viet border. The Mekong cuts across the southwest corner of Laos to exit into Cambodia(4) where it is joined by the Kong, which drains southeast Laos.
Almost two in every three people speak Lao as their first language. Less than one in ten speak Khmu. No other language accounts for as much as one in twenty. Many other Austro-Asiatic and some other Tai languages account for nearly one in four speakers. More than half the country is Buddhist.(5) As one rises in elevation animism gains importance--a contributing element among the Buddhist Lao Loum (lowlanders) and often the exclusive religion amoung Lao Theung (uplanders).(6) This religion focuses on 'phi' (spirits), including the village protector, phi ban, to whom a water buffalo is sacrificed.
There are no large cities in Laos. First place goes to the capital, Viangchan(7), on the Mekong at the Thai border. As Yung-chiang it was on Chinese trade roads from at least 128 B.C.E., although it became unimportant as maritime trade routes became pre-eminent for the neighboring empire.
The Austro-Asiatic peoples, ancesters of the Khmu and the Hmong, have been in south and southeast Asia, including Laos, a long time. Tai peoples, ancesters of the Lao, Thai and others, moved from southeast China, into Laos and Thailand, and are known from the 12th century C.E. A keynote to the change was when a Lao drove Cambodians southward in the mid-13th century and founded a kingdom.
I do not know the area's religioius roots: whether the belief in phi arrived with the Tai groups or was adopted by them; or whether Buddhism arrived with the Chinese or the Cambodians. Certainly the first Lao king was Buddhist.
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(1) Or the Lao Republic.
(2) Or Burma.
(3) Chaine Anamatique in French.
(4) Or Kampuchea.
(5) The country is communist and the voluntary or involuntary portion of non-observant people or aetheists may have risen above the one in 20 my source indicates. Despite Marxism it is still said that most young men become monks for a while, and learn to read Pali scriptures written in Lao characters.
(6) The Lao Soung (highlanders) in the north mix Animism with Confucianism.
(7) Vientiane in French.