King Louis XVI had locked the deputies of the Third Estate out of their meeting hall, Menus Plaisirs; they met instead in a nearby indoor real tennis court, where they adopted a pledge to continue to meet until a constitution had been written. 577 men signed the oath, with only one delegate refusing. This was a revolutionary act, and an assertion that political authority derived from the people and their representatives rather than from the monarch.
One week later Louis XVI called for a meeting of the Estates General for the purpose of writing a constitution for France.
The Tennis Court Oath is often considered the moment of the birth of the French Revolution.