The classical form is closely related to that of the Siamese, from which it borrows. The Lao classical orchestra can be divided into two categories, Sep Nyai (or Mahori) and Sep Noi. The Sep Nyai is ceremonial and formal music and includes: two sets of gongs (kong vong), a xylophone (lanat), an oboe (pei or salai), two large kettle drums and two sets of cymbals (xing). The Sep Noi, capable of playing popular tunes, includes two bowed string instruments, the So U and the So I, also known to the Indians. These instruments have a long neck or fingerboard and a small sound box; this sound box is made of bamboo in the So U and from a coconut in the So I. Both instruments have two strings, and the bow is slid between these two strings, which are tuned at a fifth apart and always played together. Furthermore this mahori or sep noi ensemble (the sep nyai is strictly percussion and oboe) may include several khene. In this respect, it differs markedly from the mahori orchestras of Cambodia and Siam.
Some ethnomusicologists believe that Laos is a country where the ancient art music of the Khmers has been best preserved -- as well as diverse forms of folk music related to the oldest types of Indian music, music that has largely disappeared in India itself. They claim to find in Laos a scale which the ancient Hindus called the "celestial scale," the Gandhara grama, which is a tempered heptatonic scale, or a division of the octave into seven equal parts.
Lao folk music, known as Lam, is extemporaneous singing accompanied by the khene. It is popular both in Laos and Thailand, where there is a large ethnic Lao population.
Following the Siamese conquest of Laos in 1828 and the subsequent dispersion of the Lao population into Siam (Central Thailand), Lao music became fashionable there. Sir John Bowring, an envoy from Great Britain, described a meeting with the deputy king (ouparaja) of Siam in 1855 in which his host performed on the Lao khene; at a meeting two days later he entertained Bowring with Lao dancers and khene music. The Chronicles of the Fourth Reign said the deputy king enjoyed playing the khene and "could perform the Lao type of dance and could skillfully perform the Lao comedy-singing. It is said that if one did not actually see his royal person, one would have thought the singer were a real Lao."
Immediately after the deputy king's death in 1865, King Mongkut made known his fear that Lao musical culture would supplant Siamese genres and therefore banned Lao musical performances in a proclamation in which he complained that, "Both men and women now play Lao khene (mo lam) throughout the kingdom...Lao khene is always played for the topknot cutting ceremony and for ordinations. We cannot give the priority to Lao entertainments. Thai have been performing Lao khene for more than ten years now and it has become very common. It is apparent that wherever there is an increase in the playing of Lao khene there is also less rain."