Annual or perennial herbs. Leaves opposite. Spike contracted, rarely pedunculate, capitate, terminal or axillary. Flowers bisexual, each subtended by a small bract and a pair of bracteoles; tepals 5, free; fertile stamens 2–5, staminodes usually present, all united at base into a membranous cup surrounding the ovary; stigma ± sessile. Fruit indehiscent, flattened.
About 80 species, widespread in tropical and warm-temperate areas (chiefly South America); 11 species in Australia, 5 introduced.