Perennial, usually rhizomatous, less often annual herbs, glabrous or with hairy leaf margins; rarely leaf abaxial surface with simple, many-celled trichomes. Silica bodies ±absent; oxalate raphides absent; stomata paracytic. Stem erect to ascending, rarely procumbent, usually terete, leafless or leaf-bearing. Leaves linear or filiform, spirally arranged, rarely distichous; sheath closed or open, often auriculate; auricles rarely joined to form a ligule-like structure; blades of basal leaves sometimes reduced (cataphylls). Inflorescence terminal, rarely pseudolateral (bract resembles a continuation of stem), compound, cymose or racemose, panicle-like or anthelate, many-flowered with flowers in many to one terminal heads or spike-like clusters; lower inflorescence bracts usually herbaceous; each branch with membranous bract and adaxial prophyll; flower bracteoles 1–2 or absent; rarely inflorescence reduced to a single terminal or lateral (subterminal) flower. Flowers small, usually up to 8 mm long (rarely to 40 mm), actinomorphic, hypogynous, usually hermaphrodite, rarely unisexual (dioecious or monoecious). Perianth segments 6, in two whorls, glumaceous, usually ±equal, free. Stamens 6 in two whorls or inner whorl reduced; filaments filiform to flattened; anthers 2-thecate, 4-sporangiate, oblong to linear, basifixed, dehiscence lateral; connective rarely with a projection; pollen in tetrads. Carpels 3, connate; ovary superior, unilocular or 3-septate to 3-locular; style 1, distally 3-branched (stigmas), papillae ±adaxial. Fruit an orbicular to oblong-ellipsoid loculicidal (rarely circumscissile) capsule. Seeds 3–many; endosperm starchy; embryo small, broadly cylindric; outer seed-coat hyaline, whitish to brownish, loose to adpressed, sometimes forming apical and/or basal appendages; inner seed-coat usually brown to castaneous.
Seven genera, c. 440 species, almost cosmopolitan, common from temperate to polar regions; mostly in the mountains in the tropics.
Closely related to the South African monotypic Prioniaceae (S.L.Munro & H.P.Linder, Syst. Bot. 23: 43–55 (1998)) and to Thurniaceae and Cyperaceae (D.Simpson, in P.J.Rudall et al. (eds.), Monocot. Syst. Evol. 497–509 (1995)).
Throughout the text, unless otherwise stated, the term seed is used for seeds excluding their appendages.