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.

Capsules mature late autumn--spring. Soil, sand, clay, old fields, pastures, roadside banks, temporarily moist areas; low to moderate elevations; Calif., Sask., Ont.; Calif., Iowa, Ill., Mass., N.J., Tex., Va., W.Va.; Europe; n Africa; Asia; Australia.

Although plants of this species are commonly described as three-angled, the other species may also be three-angled. The A. muticum var. rufescens from N.J., C. F. Austin "61", has reflexed apices while A. triquetrum does not always have these though illustrated as such by H. A. Crum and L. E. Anderson (1981). C. F. Austin's Musci Appalachiani 52 has a short seta and the plant is 1.2 mm, and is placed here with A. muticum var. rufescens.