On the 20th June 1789 at Versailles in France, the National Assembly swore the Tennis Court Oath in which they vowed not to separate until a written constitution had been established for the country.
Faced with enormous financial difficulties, Louis XVI had called a meeting of the Estates General that first convened in early May. This involved representatives of the three Estates – the clergy, the nobility and the non-privileged common people known as the Third Estate – meeting with the king at Versailles in an attempt to solve the economic crisis. However, the allocation of votes was unfair so the representatives of the Third Estate separated themselves from the main group and met separately. On the 13th June, by which time they had been joined by some nobles and the majority of the clergy, they declared themselves the National Assembly.
However, when the king ordered their usual meeting room to be closed and guarded by soldiers, the National Assembly feared that the king was about to force them to disband. The National Assembly instead relocated to a nearby building used for playing jeu de paume, a forerunner of modern tennis, where they swore the oath. The Tennis Court Oath therefore didn’t really happen in a tennis court, but the name has stuck.
The Oath was significant for being a collective action by French citizens against their king. Faced with such opposition Louis finally relented and, on June 27th, he ordered the remaining nobles to join the National Assembly and ended the Estates General.