| 1 | Spikelets strongly squarrose by the recurved mucros of the glumes. Small tufted annual with nuts c. ½ mm long |
| 1' | Spikelets not squarrose. Perennials, or annuals with nuts longer than ½ mm. |
| 2 | Bracts several, flat, leaf-like, not continuous with the stem. Stout perennials with well developed leaf-blades. |
| 2' | Inflorescence subtended by a single bract similar to and continuous with the stem (sometimes a very small patent second bract present), or bracts absent or glume-like. |
| 3 | Leaves all basal. Stem nodeless. Stigmas 3. Nut trigonous |
| 3' | Stems noded, leafy. |
| 4 | Glumes 1¼-2½ mm long. Nut ¾-1¼ mm long. |
| 4' | Glumes 4-7 mm long. Nut larger. |
| 5 | Perianth-bristles strongly elongating after anthesis, finally greatly exceeding the glumes, curled and entangled. Stigmas 3. Nut trigonous |
| 5' | Perianth-bristles inconspicuous, shorter than the glumes, straight, or absent. Stigmas 2 (rarely in some flowers 3). Nut planoconvex or unequally biconvex |
| 6 | Inflorescence terminal, umbelliform. Glumes pubescent outside. Bristles straight, retrorsely scabrous. |
| 6' | Inflorescence consisting of up to 4 distant corymbiform partial inflorescences. Glumes papillose or hispidulous at the top. Bristles flexuous, antrorsely scabrous |
| 7 | Inflorescence pseudolateral because of the bract looking like a continuation of the stem. |
| 7' | Inflorescence terminal, or stems and branches terminated by a single spikelet. |
| 8 | Glumes 5-7 mm long, pubescent on the back |
| 8' | Glumes at most 4 mm long, glabrous (but sometimes with ciliate margins). |
| 9 | Stem and bract transversely septate, the bract longer than the stem proper (hence inflorescence seemingly inserted in the lower half of the stem) |
| 9' | Stem and bract not septate, the bract shorter than the stem (hence inflorescence seemingly inserted in the upper half of the stem). |
| 10 | Perianth absent. |
| 10' | Perianth present, consisting of setaceous bristles or ligulate-spatulate scales. |
| 11 | Nut conspicuously transversely wavy-ridged, black when mature. Inflorescence sometimes capitate, but usually with one of the rays l(-4) cm long |
| 11' | Nut smooth, stramineous to light brown when mature. Inflorescence capitate or consisting of a single spikelet, sometimes proliferous. |
| 12 | Stems branched. Style 2-cleft, incompletely 3-cleft, and 3-cleft on the same specimen. Nuts planoconvex and triquetrous on the same specimen, 1¾-2 mm long |
| 12' | Stems unbranched. Style 3-cleft, very rarely in some flowers 2-cleft. Nuts triquetrous, usually c. 1 mm long, rarely up to 1½ mm. |
| 13 | Stamens 3. Anthers ¾-1 mm long, with distinctly produced connective. Inflorescence partly hidden by the dilated base of the bract |
| 13' | Stamens 1-2(-3). Anthers ½ mm long, with hardly produced connective. Inflorescence not hidden by the base of the bract. |
| 14 | Rhizome filiform, shortly creeping, forming mats. Stems filiform, terete, 1/5-⅓ mm thick. Leaf-blades well developed. Spikelets solitary or in clusters of 2-3 |
| 14' | Without definite rhizome. Stems tufted, strongly compressed, ½-1 mm wide. Leaves usually all reduced to their sheaths, only the upper one often with a very short, rarely ± elongated blade. Spikelets (except in depauperate specimens) in clusters of 3-12. |
| 15 | Inflorescence anthelate, open. Glumes often notched at the apex, mucronate. Connective of the anthers with a fimbriate appendage. |
| 15' | Inflorescence capitate. Glumes not emarginate, apiculate. Connective of the anthers with smooth appendage. |
| 16 | Hypogynous bristles 5-6, setaceous, retrorsely scabrous. Glumes distinctly ciliate. |
| 16' | Hypogynous scales usually 4 (3-5), ligulate-spatulate, plumosejy fringed with antrorse hairs. Glumes only microspically ciliolate |
| 17 | Stems sharply triquetrous, usually rather stout (up to 8 mm thick). |
| 17' | Stems terete or more or less 4-5-angular, but not sharply triquetrous, usually slender. |
| 18 | Involucral bract very short (1-2 cm), hardly overtopping the inflorescence. Glumes distinctly many-nerved (especially when dry). Stems usually rather coarse. |
| 18' | Involucral bract slender, 5-15 cm long, much overtopping the inflorescence. Glumes faintly nerved (only the midnerve distinct). Stems slender. |
| 19 | Bristles shorter to slightly longer than the nut, the longest 2-2½ mm. Nut biconvex (only low-convex on the ventral side). Anthers 1-1½ mm long |
| 19' | Bristles all distinctly longer than the nut. Nut planoconvex (the ventral side flat). Anthers ½-¾ mm long |
| 20 | Spikelets 1(-2)-flowered. Submerged very delicate aquatic, the stem-nodes with numerous thread-like sterile stems not to be confused with leaves |
| 20' | Spikelets more-flowered. Habit quite different, if an aquatic each node with one true leaf. |
| 21 | Leaf-blades well developed. Perianth-bristles absent. Stigmas 2; nut planoconvex or biconvex. Stems procumbent or floating, branched, the branches bearing a single terminal spikelet. |
| 21' | Leaf-blades reduced to an up to 2 cm long (usually much shorter) mucro. Perianth-bristles 6. Stigmas 3; nut trigonous. Stems erect, not branched. Inflorescence consisting of 1-5 spikelets. |
| 22 | Nut planoconvex, sharply angled, 1¼-1½ by c. 1 mm. Spikelets 3-6 by 2-3 mm |
| 22' | Nut slightly biconvex in the centre, thinner towards the margins and there thickened into an obtuse edge, 1¾-2 by c. 1 mm. Spikelets 5-8 by 3-4 mm |
