Herbs shrubs or subshrubs, erect or procumbent; stems with swollen nodes, rooting at lower nodes, quadrangular, longitudinal dark lines or grooved on two opposite. Leaves simple, opposite decusssate or subalternate (seemingly alternate due to disappearance of one leaf of each pair); cystoliths conspicuous or not apparent; petiolate or rarely sessile. Inflorescences axillary or terminal, simple spike or branch at base, dichasium cyme or laxly paniculate, occasionally reduced to condensed, cluster of flowers or rarely solitary flower; 1–6-flowered on each side of inflorescence node, flower subtended by one bract and two bracteoles; bracts triangular, lanceolate, spathulate, ovate or orbicular (except in simple diachasia bract absent); bracteoles smaller than bracts. Calyx deeply 4–5-lobed, equal in size. Corolla tubular and bilabiate, 2-lipped, upper lip erect, tapering, emarginate and lower lip spreading, 3-lobed, usually with colored markings on lower lip, tube usually pubescent at vein bases, tube shorter or longer than lip. Stamens 2, inserted near middle of corolla tube, shorter than upper lip; anthers 2-thecous, thecae equal or unequal in length, 1 or both with a basal appendage (tail) or appendage absent, staminodes absent. Ovary superior with 2 ovules per locule; disc annular or shallowly cupuliform; style terminal, exserted; stigma minutely, unequal lobes. Fruits 2 valved, clavate, loculicidal capsule; capsule ovoid or oblong, placenta fixed or rising elastically from base. Seeds 4 per capsule, supported by retinacula.

About 600 species, mainly distributed throughout pantropical. Forty eight species and fifty taxa are recognised in Thailand.