The Hmong are an Asian ethnic group from the mountainous regions of China, Vietnam, Laos, and Thailand. Hmong are also one of the sub-groups of the Miao ethnicity in southern China. Hmong groups began a gradual southward migration in the 18th century due to political unrest and to find more arable land. The main traditional Lao instrument is the flute-like khene (kaen) which is made of bamboo. The khene (also spelled khaen, kaen and khen) is a mouth organ of Lao origin whose pipes, which are usually made of bamboo, are connected with a small, hollowed-out hardwood reservoir into which air is blown, creating a sound similar to that of the violin. Today associated with the Lao of Laos and Northeast Thailand, similar instruments date back to the Bronze Age of Southeast Asia. In Cambodia, it is used among the ethnic Lao population of the province of Stung Treng and is used in lakhon ken, a Cambodian dance drama genre that features the khene as the premiere instrument.