A postilion (or postillion, occasionally Anglicised to "post-boy" rider was the driver of a horse-drawn coach or post chaise, mounted on one of the drawing horses. By contrast, a coachman would be mounted on the vehicle along with the passengers.
Postilion riders normally rode the left (or "near") horse of a pair because horses usually were trained only to be mounted from the left. With a double team, either there would be two postilions, one for each pair, or one postilion would ride on the left rear horse in order to control all four horses.
Postilions were typically supplied with a special rigid boot for use on their inside (right hand) leg. This appliance provided protection from possible crushing injury due to contact with the central wooden shaft (if any) and the body of the adjacent horse.
This style of travel was known as "posting." The postilions and their horses (known as "post-horses") would be hired from a "postmaster" at a "post house." The carriage would travel from one post house to the next (a journey known as a "stage"), where the postilions and/or spent (exhausted) horses could be replaced if necessary. In practice unless a return hire was anticipated a postilion of a spent team frequently was also responsible for returning them to the originating post house.