brass musical instrument of cylindrical bore, used by guards of mail coaches in the 18th and early 19th centuries. At the end of the 18th century, post horns were crescent-shaped, coiled, or straight. The notes they sounded were at most six (harmonics 2 to 7). The post horn gave rise to the cornet in the 19th century, when valves were applied to it.

The coach horn, which was like a straight post horn, though longer, was made of copper and was of conical bore. It was used on the London–Oxford mail coach until 1914.