Tufted or rhizomatous perennial or occasionally annual herbs, glabrous. Leaves mostly basal; sheath open; blade flat, channelled, terete and stem-like, solid or hollow and internally septate, or reduced to a mucro surmounting the leaf-sheaths (in which case the leaves are referred to as cataphylls). Inflorescence mainly cymose, the cymes contracted into clusters or expanded into panicles, apparently terminal or lateral to the primary bract that sometimes appears as a continuation of the culm; flowers closely subtended by 2 (rarely more) small hyaline prophylls (bracteoles), lacking in some species. Flowers bisexual or rarely unisexual and plants dioecious; tepals of the outer whorl usually slightly longer and more rigid, with narrower membranous margins than those of the inner whorl; stamens 3–6. Seeds small, numerous, shortly apiculate or appendaged at one or both ends.
About 310 species, cosmopolitan; 72 species in Australia (48 native). The following points should be borne in mind when using the keys:
Colour: For species with leaves that are reduced to cataphylls (section Juncotypus) the colour of the culms and cataphylls is based on fresh material. The colour of cataphyll bases refers to non-submerged cataphylls, as cataphylls in any of the taxa may be blackish if long-submerged in water.
Culm diameter, hardness and striation number: Culm diameters in keys and descriptions are measured about halfway along the length of the culm. Assessment of culm hardness and counts of the number of vertical striations are also made at this point.
Pith: Pith should be examined in at least two parts of the culm, as in some species the pith is variable. Pith is best viewed by making a diagonal-longitudinal slice towards the centre of the culm with a razor blade.
Stomates and epidermal cells: The position of the stomates in relation to the epidermis, and the degree of radial elongation of the epidermal cells overlying the striations are important characters for some species. Thin transverse sections are satisfactory for examining these characters and should be taken at a third to halfway from the culm base with a sharp razor blade, and then examined under low to medium power of a compound microscope.
Tepals: Tepal measurements apply to the outer tepals with intact apices. Tepals are most precisely measured after detachment and flattening. Tepal colour is based on material prior to capsule dehiscence.
Capsules and seeds: Capsule measurements apply to the length of the central axis of ripe but unopened capsules. Seed measurements include appendages.
Difficult specimens: Very depauperate specimens of species in section Juncotypus (with reduced vegetative organs and inflorescences) are occasionally encountered. Such specimens may be difficult to determine. Hybrids are also not uncommon amongst species in this group, and in some cases hybridization may also account for difficulties experienced with identification.