On June 20, 1789, the French National Assembly (members of the Third Estate) went to the meeting chambers of France, and found the chamber doors were shut and locked.
"Not to separate, and to reassemble wherever circumstances require, until the constitution of the kingdom is established.
They thought the king was going to get rid of the National Assembly, so they went to a nearby court and took the Tennis Court Oath.
The Third Estate began finding new places to meet until a new form of government was created that was fair and just.
The Oath signified for the first time that French citizens formally stood in opposition to Louis XVI.
The National Assembly's refusal to back down forced the king to make concessions. It inspired a wide variety of revolutionary acts in the months afterwards which reinforced the assembly's strength and Louis reluctantly requested that votes be based on head counts, not each estates' power.