1 Scales awned or cuspidate, cusp 0.3-0.5 mm long, excurved; achenes 0.6 mm long; [section Pumili]

1 Scales obtuse to slightly mucronate, apex not notably excurved; achenes (0.7-)1-1.6 mm long.

  2 Floral scales divaricate (thus the spikelet margin appearing coarsely dentate), with whitish or hyaline margins; coarse, erect plants 30-75cm tall, inflorescence diffuse; [section Albomarginati]

  2 Floral scales with tips appressed (proximal sometimes spreading in C. polystachyos), lacking hyaline border; plants mostly smaller.

    3 Achenes oblong, subterete, often only slightly compressed, apex truncate, apiculate; scales 3-5-nerved; spikelets mostly <2 mm wide (except C. filicinus); [section Pycreus].

      4 Scales 2.5-3.6 mm long, 1.6-1.8 mm wide; achenes 1.2-1.6 mm long, 0.6-0.9 mm wide, narrowly obovoid; [beaches, maritime habitats]

      4 Scales 1.3-2.4 mm long, 1.0-1.4 mm wide; achenes 0.8-1.2 mm long, 0.4-0.6 mm wide; [habitats various]

        5 Cespitose annual, < 12 cm tall; floral scales ovate, 1.3-1.6 mm long, loosely imbricate; styles 0.1-0.2 mm long; stigmas 0.3-0.4 mm long; filaments 1.2-1.4 mm long; anthers 0.1-0.2 mm long; achenes stipitate, strongly compressed laterally; [LA and westwards, rare]

        5 Shortly rhizomatous perennial, mostly > 12 cm tall; floral scales oblong, 1.8-2.4 mm long, closely imbricate; styles 0.6-1 mm; stigmas 1.4-2 mm long; anthers 0.6-0.8 mm long; achenes substipitate to cuneate, slightly compressed laterally; [widespread]

    3 Achenes ovoid, ellipsoid, or obovoid, biconvex, apex rounded to acute; scales bicarinate medially; spikelets mostly > 2mm wide.

          6 Achenes black to dark reddish brown, with encrusted transverse whitish lines, epidermal cells longitudinally elongate, rectangular; scales pale brown or yellowish, the dark achenes strongly contrasting and visible through the spikelets in life (without backlighting); fibrous-rooted annuals; [section Zonati].

          6 Achenes black to brownish, without transverse lines, epidermal cells square or isodiametric; plants either perennials or scales darker.

             7 Scales uniformly pale yellowish to yellowish brown; cespitose perennials, rhizomes short; [section Propinqui].

               8 Anthers ellipsoid, 0.3-0.6 mm long; spikelets 1.8-3 mm wide, with scales 1.8-2.6 mm long; achenes brown, ellipsoid, ovoid, or obovoid, 0.4-0.8 mm wide; [more widespread]

               8 Anthers linear, 2-2.4 mm long; spikelets 3-5 mm wide, with scales (3.6-) 3.8-4.2 (-4.6) mm long; achenes black, broadly obovoid to orbiculate, 0.8-1 mm wide; [s. peninsular FL, rare waif]

             7 Scales with anthocyanic (reddish-brown or purplish) pigmentation; fibrous-rooted annuals (except C. sanguinolentus); [section Vestiti].

                 9 Floral scales firmer, subcoriaceous, not bilaterally sulcate; reddish pigmentation ±uniformly distributed or concentrated basally and medially; floral scales 2.5-3 mm long; annuals, culms usually <25 (35) cm long

                 9 Floral scales membranous, each side with a narrowly elliptic, shallow, pale, translucent groove outlined at least marginally by reddish pigmentation; floral scales 1.8-2.7 mm long; annuals or perennials.

                   10 Rhizomatous perennials, culms usually > 25 cm tall; stamens 3; style divided less than half its length; style branches not so conspicuously exserted, exposed portions less than half as long as floral scale