Unfit
In 19811, the King was thought to be an unfit ruler and stepped aside. George III might have been a popular king, but he was suffering from severe illness. Even though he was feeling well enough to agree to the regency, he went completely crazy and failed to recover by the end of the year. His son and Prince Regent went on to reign in his place until his father died in 1820. The younger man is King George IV.
Regency
The idea of regency was brought up earlier in the reign of George III. In 1788, he was suffering from a spell of insanity that made him unable to give the speech for the opening of the parliament. This led to a crisis and paralyzed the United Kingdom’s government. Politicians then agreed that they needed a regent, although the King was able to recover before they could pass the necessary legislation.