Annual or perennial herbs, or shrubs or small trees, rarely small, woody climbers. Leaves simple, alternate, rarely opposite or whorled, petiolate; leaf blade papery or subleathery, margin entire, glabrous or pilose. Racemes terminal, axillary, or extra-axillary. Flowers bisexual, zygomorphic; bracts 1-3, persistent or caducous; bracteoles present. Sepals 5, unequal, persistent or caducous, in 2 ranks; outer 3 small; inner 2 petaloid, large. Petals 3, connate in lower 1/2, white, yellow, or purple-red, keel boat-shaped, cucullate, or helmet-shaped enclosing stamens and style, apex with often highly divided appendage. Stamens 8; filaments united at least at base, forming trough-shaped sheath ("staminal sheath"), open on upper side, adnate with petals, variously united or free above; anthers basifixed, with or without stalk, 1- or 2-celled, dehiscent from terminal pore or slit. Disk present or absent. Ovary 2-loculed, compressed; ovule 1 per locule, anatropous, pendulous; style 1, erect or curved; stigmas 1 or 2. Fruit capsular, compressed, often winged. Seeds 2, usually black, ovoid, rounded or shortly cuneate, pubescent or glabrous, strophiolate at hiliferous end, strophiole cap-shaped, cucullate, margin entire or variously lobed, other end sometimes with appendage.

About 500 species: widespread worldwide; 44 species (21 endemic) in China.