The Hall of Union was originally built in 1420 and reconstructed in 1655. It is the smallest of the three rear palaces in the Inner Court, and corresponds to the Hall of Complete Harmony in the Outer Court. This hall was used for the Empress' birthday celebrations.

There are 25 caskets with covers here made to contain the imperial seals. The imperial seals have been kept here since the reign of Emperor Qianlong. Now, for the convenience of visitors, the imperial seals have been taken out of their caskets to be on display in glass cases located on either side of the door.

The large chiming clock on the left hand side was built in the Palace in 1798 during the reign of Emperor Qianlong, and miraculously, the clock still works.

The water clock on the right hand side was built in 1745. It may be one of China s most outstanding inventions which dates back about 2,500 years. It wasn't until the mechanical clock was introduced to China after the reign of Emperor Qianlong (1736-1795) that the use of the water clock was abandoned altogether.

Most conspicuous of all is that white tablet at the far end, which bears two Chinese characters "Wu Wei", literally, "to refrain from action" or "laissez faire". This reflects the philosophy of Taoism a religion which is indigenous to China and used often by the feudal rulers to discourage people from taking any action. Its most famous teaching means not to take any action is to take action.