Annual to perennial, tufted or mat-forming herbs, glabrous, subglabrous or glandular-hairy. Leaves opposite; stipules absent; bracts scarious with broad, green midrib. Flowers in dichasia or solitary and terminal, usually 5-merous, bisexual; sepals 5, free; petals entire to slightly emarginate, sometimes absent; stamens 10; ovary 1-celled, with few to many ovules, styles 3, rarely more. Capsule ovoid or oblong, opening by as many teeth as styles; seeds 1–many, subdiscoid to nautiloid, tuberculate or minutely echinate, rarely smooth.
About 120 species, extending from Arctic regions to Mexico and Chile, north-east Africa, and Himalaya; 1 naturalised in Australia.