This emblematic place was the theatre of the oath during which 578 deputies of the States General swore not to be separated until they had given a constitution to France. The French Revolution was begnning...
During the Revolution, the idea was raised to replace the Royal Tennis court with a commemorative monument, but nothing was done before 1880, when the hall was turned into a museum in preparation for the centenary of that decisive event.
Make sure you wear something warm, as the room is not heated!