A widely distributed but mainly temperate genus, with about 300 species of annuals and perennials, the latter mainly herbaceous but a few with persistent, more or less woody stems. Leaves alternate, with sheathing stipules. Flowers usually in spikes, racemes or panicles. Perianth segments five (sometimes four or six), in two whorls, those of the outer whorl winged on the back in some species. Stamens five, sometimes more or fewer. Style with two or three stigmas. Fruit a three-sided or lens-shaped nutlet, enclosed by the persistent perianth.