The Thammayut order is the reform half of Thai Buddhism, and it is unusual in having been founded by a future king during his own years as a monk.
The reform of Mongkut
Prince Mongkut, later King Rama IV, spent twenty-seven years as a monk before taking the throne, and during that time he sought to strip Thai monasticism of syncretic and lax practice and to ground it strictly in the Pali Canon and a rigorous reading of the Vinaya, adopting the Mon ordination line he judged more authentic. The order he founded in 1833, whose name means the order adhering to the Dhamma, centres on Wat Bowonniwet in Bangkok and was favoured by the royal house.
The smaller of two official orders
After the Sangha Act of 1902 the Thammayut became one of the two official monastic orders of Thailand, the smaller and more disciplined alongside the far larger Maha Nikaya, and it spread its ordination line to Cambodia and Laos. Its royal origin and its rigorist self-understanding give it an influence out of proportion to its size, and the coexistence of a small strict order and a large mainstream one is the basic structure of the Thai sangha to this day.