1 Culms sharply trigonous in cross-section (the faces concave), and harshly scabrid on the angles (very rarely smooth); leaf blades, sheaths and bracts nodulose, with cross-ribs between veins.

  2 Inflorescence congested, primary rays 3-5, secondary and tertiary rays absent; scales ovate, 1.4-1.6 mm long; spikelets 1.5-2.2 mm wide, in dense globose clusters; achenes 1.0-1.2 mm long, < 0.7× as long as the subtending floral scale

  2 Inflorescence broad, diffuse, primary rays 6-12, secondary and often tertiary rays present; scales oblancolate, (1.3-) 1.5-2.0 (-2.4) mm long, spikelets 2-3.2 mm wide, in hemispheric to irregularly globose clusters; achenes 1.2-1.5 mm long, at least 0.7× as long as the subtending floral scale

1 Culms obtusely trigonous in cross-section (faces usually convex), not scabrid on the angles, sometimes scabrous across faces; leaf blades and bracts not nodulose.

    3 Bracts 1-3, < 3 mm wide, the largest erect, appearing as a continuation of the culm; culms slender, 1-2 mm in diameter; plants cespitose, fibrous-rooted annuals (perennial in C. reflexus).

      4 Scales yellowish green or straw-colored, sometimes with an orange tinge, apices incurved to straight; stems and rays scabridulous with retrorse prickles, rough to the touch (rarely with only a few prickles); spikelet clusters mostly loosely and irregularly fasciculate.

      4 Scales either dark reddish-brown laterally, often with contrasting green keels or mucronate and conspicuously excurved apically; culms glabrous (rarely scabridulous in C. acuminatus); spikelet clusters densely globose to ovoid or pyramidal.

        5 Scales slightly to strongly excurved apically, mucronate, greenish or stramineous, or sometimes light orange-brown laterally, but this grading evenly into the greenish or yellowish medial portion; achenes papillose, broadly ellipsoid; plants annual, rhizomes absent; heads hemispheric to globose

    3 Bracts (3-) 6-12, the larger > 3 mm wide, spreading to ascending; culms coarser, 2-5 mm in diameter; plants rhizomatous or cespitose perennials.

          6 Achene with whitish spongy basal thickening

          6 Achene without basal thickening.

             7 Spikelets elliptic to orbiculate-ovate, ≤ 5 mm long, very densely packed in subglobose, lobulate heads; achenes linear, 4-5× as long as wide, falcate; scales oblong-spatulate, (1.5-) 2-2.5 mm long, apices often excurved

             7 Spikelets lanceolate to linear, usually > 5 mm long, heads usually looser; achenes narrowly to broadly ovoid or ellipsoid, not falcate, < 4× as long as wide; scales ovate to lanceolate, to 2 (-2.3) mm long, apices straight to incurved (excurved in C. eragrostis).

               8 Scales declined 60-90 degrees from the rachilla, blunt, incurved; achenes ellipsoid, < 3× as long as wide, narrowed to a distinct stipe; achene surfaces distinctly honeycomb-reticulate, often iridescent

               8 Scales declined 30-45 (-60) degrees from the rachilla; achenes very narrowly ellipsoid, 3-4× as long as wide, stipe absent to poorly developed (ellipsoid with distinct stipe in C. eragrostis); achene surfaces dull, smooth to puncticulate or obscurely reticulate.

                 9 Spikelets > 3 mm wide; scales 2-2.3 mm long, excurved apically; heads 20-40 mm in diameter, densely globose, lacking secondary rays; achenes broadly ellipsoid, < 2.5× as long as wide, with a distinct stipe

                 9 Spikelets < 3 mm wide; scales < 2 mm long, incurved to straight; heads mostly ≤ 20 mm in diameter, usually with secondary rays; achenes narrowly ovoid or ellipsoid, 3-4× as long as wide, stipe absent.

                   10 Culms smooth (or rarely with sparse retrorse prickles); plants robust perennials, with thick deeply set rhizomes; culms mostly >5 dm tall, bases purplish black; floral scales diverging about 30 degrees from the rachilla, pale green; heads usually tightly globose

                   10 Culms scabrid (at least apically) with abundant retrorse prickles; plants short-lived cespitose perennials, rhizomes absent; culms mostly < 5 dm tall, bases reddish or brownish; floral scales usually diverging at least 45 degrees from the rachilla, yellowish to yellowish-green; heads mostly loosely and irregularly fasciculate, with flattened spikelets fanned out like a deck of cards