Glabrous herbs. Stems terete at base to quadrangular or slightly 4-winged at top, occasionally branched. Leaves decussate, sessile, simple, without stipules, arranged along stem; lower leaves usually smaller than upper ones; blade green, rarely glaucous, margins entire, base semi-amplexicaul; primary vein distinct, secondary veins sometimes distinct. Inflorescences 1-10-flowered modified dichasia, usually terminal, rarely in leaf axils, or flowers solitary; bracts and bracteoles leaf-like. Flowers actinomorphic, 4-merous, erect to slightly nodding, sessile to pedicellate; calyx green, rarely yellowish or maroon, 4-winged or 4-ribbed, connate for at least 1/3 of its length, lobes linear-triangular to triangular, entire, acute-acuminate; corolla funnel- to slightly salver-shaped, 4-lobed, connate for at least 1/2 of its length; stamens 4, inserted on corolla tube, filaments straight, with or without dentate appendages at base, anthers introrse, dorsifixed, straight at anthesis; pollen in tetrads; ovary 1-locular, placentation parietal, sometimes surrounded by a dark disk-like ring at base, ovules many, style 1, filiform, as long as or longer than corolla tube, stigma 2-lobed. Fruit a septicidal capsule with persistent calyx and usually persistent corolla; seeds many, small, variable in shape, elliptic to almost globose.