The post horn is a valveless cylindrical brass instrument with a cupped mouthpiece. The instrument was used to signal the arrival or departure of a post rider or mail coach. It was used by postilions of the 18th and 19th centuries.

A mail coach is a stagecoach that is used to deliver mail. In Great Britain, Ireland, and Australia, they were built to a General Post Office-approved design operated by an independent contractor to carry long-distance mail for the Post Office. Mail was held in a box at the rear where the only Royal Mail employee, an armed guard, stood. Passengers were taken at a premium fare. There was seating for four passengers inside and more outside with the driver. The guard's seat could not be shared. This distribution system began in Britain in 1784. In Ireland the same service began in 1789, and in Australia it began in 1828.