Every freeman had to be registered as a servant, or phrai, with the local lord, or nai, for military service and corvee labor on public works and on the land of the official to whom he was assigned. The phrai could also meet his labor obligation by paying a tax. If he found the forced labor under his nai repugnant, he could sell himself into slavery to a more attractive nai, who then paid a fee to the government in compensation for the loss of corvee labor. As much as one-third of the manpower supply into the nineteenth century was composed of phrai.
Wealth, status, and political influence were interrelated. The king allotted rice fields to governors, military commanders, and court officials in payment for their services to the crown, according to the sakdi na system. The size of each official's allotment was determined by the number of persons he could command to work it. The amount of manpower a particular nai could command determined his status relative to others in the hierarchy and his wealth. At the apex of the hierarchy, the king, who was the realm's largest landholder, also commanded the services of the largest number of phrai, called phrai luang (royal servants), who paid taxes, served in the royal army, and worked on the crown lands. King Trailok established definite allotments of land and phrai for the royal officials at each rung in the hierarchy, thus determining the country's social structure until the introduction of salaries for government officials in the nineteenth century.