Plants perennial. Culms to 150 cm, erect or decumbent, rooting at the nodes, usually branched above the bases. Leaves not aromatic; sheaths mostly glabrous, sometimes ciliate near the base; ligules membranous, ciliate; blades usually linear-lanceolate, sometimes linear. Inflorescences terminal and axillary, with 1(2) flattened rames borne on a common peduncle, spikelets partially embedded in the rame axes; disarticulation in the rames, usually oblique and often tardy. Spikelets in heterogamous sessile-pedicellate pairs, dorsally compressed. Sessile spikelets with 2 florets; calluses blunt; lower glumes coriaceous, smooth; upper glumes equaling the lower glumes, chartaceous to membranous, sometimes partially adnate to the rame axes, sometimes awned; lower florets reduced to hyaline lemmas; upper florets bisexual, lemmas unawned. Pedicels thick, fused to the rame axes. Pedicellate spikelets morphologically similar to the sessile spikelets, staminate or sterile. x = 9, 10. Name from the Greek hemi, half, and arthron, segment, because of the tardily disarticulating rames.