aequus

Aequus, aequa, aequum is a first and second declension adjective whose senses all run off "level". Level ground is aequus; two things at the same level are equal; a judge who holds them level is fair; a mind that stays level is calm. Aequo animo, "with an even mind", is the phrase behind English equanimity, and the neuter aequum est means "it is right that".

The trap is aequus against equus, "horse". One vowel, no relation, and medieval manuscripts that flatten ae to e make them identical on the page: equus in a chronicle may be either. Context decides. The family is large: aequalis, aequitas, aequo, aequor, and English equal, equity, adequate, iniquity and the equator, which is simply the circle that levels the globe.