In the early phase of the French Revolution, members of the Third Estate, the commoners who made up most of France's population, which was divided into a series of "estates" denoting one's place in the social hierarchy (the Fourth Estate was comprised of the peasantry), signed a pledge, or oath, denouncing their exclusion from a key meeting of the Estates-General in June 1789. This meeting was the first of the revolution constituting a general assembly, and its exclusion by the First and Second Estates (the clergy and the nobles, respectively) resulted in the Third Estate's pledge to form a National Assembly to usurp the position in the revolution of the Estates-General.
The Oath of the Tennis Court got its name from the indoor tennis court where the Third Estate members gathered to take the oath "...never to separate ourselves from the National Assembly, and to reassemble wherever circumstances require, until the constitution of the realm is drawn up and fixed upon solid foundations."
[The text of the oath refers to the requirement "...to meet in the Tennis Court of Old Versailles street..."]
The Oath of the Tennis Court was instrumental in fomenting a popular revolution in France, rather than one controlled by vested powers.