Plants: three-angled, about 1 mm. Stem: leaves cuspidate, keeled; laminal cells, 13–18 µm wide, 1–3:1, smooth. Seta: as long as the diameter of the capsule. Spores: 25–30(–40) µm, finely papillose. Phenology: Capsules mature late autumn-spring.
Soil, sand, clay, old fields, pastures, roadside banks, temporarily moist areas. low to moderate elevations. Ont., Sask., Calif., Ill., Iowa, Mass., N.J., Tex., Va., W.Va., Europe, Asia, n Africa, Australia.
Acaulon muticum var. rufescens has reflexed apices while A. triquetrum does not always have these, though illustrated as such by H. A. Crum and L. E. Anderson (1981). The exsiccat C. F. Austin, Musci Appalachiani 52 has a short seta and the plant is 1.2 mm, and is placed here with A. muticum var. rufescens.