An order of monocotyledonous angiosperms composed of a single genus, Acorus (sweet flag or sweet calamus), with two species widespread in the Northern Hemisphere. Formerly, Acorus was included in the aroid family, Araceae, but several lines of evidence, including deoxyribonucleic acid (DNA) sequences, have firmly established it as the first-branching lineage of the monocotyledons. These species are emergent aquatics with peculiar inflorescences, and they are unusual among the monocots in having characters otherwise confined to the magnoliid dicots, such as the anther formation and the presence of ethereal oils. These oils are the basis of the species' frequent use as a medicine to treat toothache and dysentery, and as an ointment in religious ceremonies.