Plants forming turfs or cushions, light to blackish, olive or reddish green distally, brown to reddish brown or tan proximally. Stems to 2(-9) cm; hyalodermis occasionally present, sclerodermis usually present, central strand usually distinct; axillary hairs ca. 5 cells long, proximal 1-2 cells brownish. Leaves crowded, appressed-incurved, occasionally twisted or curled when dry, spreading to strongly reflexed and occasionally keeled when moist; ovate to long-lanceolate or long-triangular, adaxial surface usually broadly concave, occasionally narrowly channeled, 0.4-3(-6) mm; base weakly differentiated in shape to oblong and half-sheathing the stem, proximal margins occasionally decurrent; distal margins plane to recurved or revolute, entire or occasionally weakly dentate or crenulate, occasionally 2-stratose in patches or entirely; apex narrowly acute to rounded, occasionally fragile or caducous; costa ending several cells before apex to short-excurrent, adaxial cells quadrate to elongate, in 2-4(-8) rows; transverse section ovate, semicircular or reniform, adaxial epidermis present or seldom absent (rarely bulging as a 1-stratose pad of cells in D. nevadensis), adaxial stereid band usually weak, occasionally absent in small plants or entirely in D. australasiae, D. revolutus and D. umbrosus, guide cells 2-6 in 1(-2) layers, hydroid strand occasionally present, abaxial stereid band weak to strong, crescent-shaped or ovate in sectional shape, abaxial epidermis present but usually weak; basal laminal cells not or weakly differentiated to strongly differentiated across leaf or rising higher medially, quadrate to rectangular, seldom bulging, usually little wider than distal cells, ca. 2-4:1, walls of basal cells usually rather thin, occasionally porose or transversely slit (D. umbrosus), smooth to papillose; distal medial cells subquadrate to hexagonal or rounded-angular, occasionally shortly rectangular or rhomboid, usually 8-13 µm wide, 1:1, 1-stratose, papillae usually simple to 2-fid, usually solid, with 3-5 projections, occasionally absent or multiplex, cell walls thin to thick, lumens sometimes angular, occasionally trigonous (D. giganteus), weakly to strongly convex on both surfaces. Specialized asexual reproduction by gemmae occasional, usually spheric to elliptic, of 1-10 cells, usually borne in leaf axils, occasionally on adaxial surface of costa, or rarely as small tubers borne on rhizoids in the soil. Sexual condition dioicous (occasionally possibly rhizautoicous). Perichaetia terminal, interior leaves not or occasionally sheathing in proximal 1/2, ovate to long-lanceolate, occasionally somewhat enlarged, laminal cells rhombiodal-rectangular in proximal 1/2. Seta yellowish to reddish brown, mostly 0.3-2 cm, twisted clockwise proximally, occasionally counterclockwise distally. Capsule stegocarpous, theca elliptic to cylindric, ca. 1-3 mm, annulus of 1-3 rows of hexagonal, often vesiculose cells, often deciduous in pieces or revolute; operculum short- to long-conic or conic-rostrate, ca. 0.5-1.2 mm, cells in straight rows or twisted counterclockwise; peristome teeth 32, or 16 and grouped in pairs, occasionally absent or rudimentary, oblong to linear or long-triangular, often medially perforated or cleft, usually straight or weakly twisted counterclockwise, teeth to 700(-1300) µm, sometimes rudimentary. Calyptra cucullate. Spores ca. 7-18 µm. KOH laminal color reaction red, occasionally red-orange or yellow.
Species ca. 122 (26 in the flora): worldwide.
The genus Didymodon is distinguished (K. Saito 1975; R. H. Zander 1978d) from its closest relative, Barbula, by its usually lanceolate to long-lanceolate leaves, axillary hairs with 1 or more brown proximal cells, basal laminal cells usually little differentiated from the distal, abaxial costal cells usually quadrate, laminal papillae absent or simple or only occasionally multiplex, gemmae composed of only 1-10 cells, and peristome teeth seldom twisted, occasionally absent or rudimentary. Recent treatments of the genus or its sections in the flora area or Mexico include those by H. Robinson (1970), W. C. Steere (1938b) and Zander (1978d, 1981b, 1994k). The leaves of most species of Didymodon are yellow or orange in nature (before application of KOH), unlike those of Bryoerythrophyllum, which are red.