Annual or perennial herbs, rarely parasitic or saprophytic. Stem erect or procumbent, simple or branched from the base, usually 4-angled, angles smooth, winged or not; sometimes with highly congested internodes. Leaves well developed or extremely reduced and scale-like (in saprophytic and parasitic taxa), opposite, sessile, shortly petiolate or amplexicaul. Inflorescences terminal, flowers solitary or cymose, sometimes dichotomously or dichasially branched. Flowers pedicellate, 4–5-merous, sometimes heterostylous, regular. Calyx lobes free to the base or sometimes forming a short tube, linear-lanceolate to ovate or obovate, dorsally winged; wings scabrid-denticulate, acute. Corolla contorted in bud; tube funnel-shaped or cylindric; lobes ovate, obovate or elliptic. Filaments inserted on the corolla tube or in the sinuses of the corolla lobes; anthers often oblong, exserted or included, with or without 1 stipitate apical and two minute basal glands. Ovary ovoid or obovoid or ± globose, bilocular; style exserted or included, often with swelling (secondary stigma) at the base or middle of its length, or long or short in the same taxon; stigma capitate or clavate, bilobed or entire. Capsule ovoid or obovoid, membranous or coriaceous, bivalved, dehiscence septicidal. Seeds minute, numerous, surface pitted, ± slightly reticulate or with ridge-like outgrowths.