A cornu or cornum (Latin: cornū, cornūs or cornum, “horn”, plural cornua, sometimes translated misleadingly as “cornet”) was an ancient Roman brass instrument about 3 m (9.8 ft) long in the shape of a letter 'G'. The instrument was braced by a crossbar that stiffened the structure and provided a means of supporting its weight on the player's shoulder. Some specimens survive in the archaeological record, two from the ruins of Pompeii. (source)