The National Assembly (1789) was the body of representatives that broke away from the Estates-General on June 17, 1789, proclaiming themselves as an independent political body, during the French Revolution. They called themselves National Assembly because they considered that they represented the nation. As they were not allowed to be in session in the State Hall of Versailles in the Estates-General meetings, they gathered in a tennis court and swore a solemn oath on June 20, vowing never to separate until the approval of a Constitution.
When the Estates-General sessions began on May 5, the members of the National Assembly constituted the Third Estate, which represented the commoners: the bourgeoisie (merchants, lawyers, journalists), the craft-men, workers, and farmers. The Estates-General were composed of the three social classes France was divided into: the clergy, nobility, and commoners; they had been convened by Louis XVI, in 1788, to help solve the financial crisis which had arisen due to the high debt.
The main reason for this political schism within the Estates-General was the disagreement over the voting system when it came to making a decision; the nobility members wanted one vote by estate, whereas the commoners wanted one vote per deputy. With one vote by estate voting system, the nobles and the clergy (the privileged classes) would always win, passing resolutions that would favor them. On the other hand, with one vote per representative, the commoners would win as they were twice as many as the other two estates.
The second reason for the separation of the Third Estate members and the formation of the National Assembly was that the commoners, especially the bourgeoisie, wanted deep political and tax reforms, too. Let us bear in mind that lawyers, writers, and journalists had been influenced by the Enlightenment political and philosophical movement of the 18th century and they wanted political and social equality.
Below, the National Assembly members swearing the Tennis Court Oath. Painting by Jacques-Louis David