cornu

Cornu, cornus is fourth declension and neuter, which is unusual enough that it is worth memorising outright: the whole singular is cornu except the genitive, and the plural runs cornua, cornuum, cornibus, cornua, cornibus. Genu (knee) and veru (spit) behave the same way.

The senses run from the animal's horn to anything made of horn - a drinking vessel, a trumpet, the end of a bow - and then to the flank of a battle line, which is the sense that fills military prose: cornu dextrum, cornu sinistrum. The trap is declining it like a second declension neuter; there is no form cornum. It is cognate with English horn and with Greek keras, and gives cornet, corner, cornucopia and unicorn.