The term 'Exon', formerly spelt 'Exempt' derives from the French work indicating when an officer of the Life Guards has been freed from duty, or was taking the place of the Captain in charge of the night watch. Four such Exons were attached to the Yeoman of the Guard, taking it in turn to command them at St. James's Palace and living at the Palace while doing so.

The uniform worn by Exons is similar to that of other officers, a type of General's uniform with epaulettes; the Captain wears Gold aiguillettes from the right shoulder.