Annual, decumbent to ascending herbs; glabrous to glandular-hairy. Leaves opposite, but pseudo-verticillate owing to axillary fascicles; stipules small, not connate around the node, deciduous; bracts small, scarious. Flowers in loose, terminal dichasia; bracts small, scarious. Flowers 5-merous, bisexual; sepals 5, free; petals entire; stamens 5–10; ovary 1-celled, with several ovules, styles 5. Capsule ovoid to subglobose, splitting deeply into 5 valves; seeds several, lenticular-discoid to subglobose, winged or sharply keeled, often white-papillose.
5 species, native to the temperate Northern Hemisphere; 2 species naturalised in Australia.