Plants of P. commune form loose to dense turfs in locally frequent to dominant colonies. The wiry plants are large and olive to dark green, becoming brown with age. Stems are typically unbranched, 2-25 cm high, stiff, erect or decumbent (lying down with ascending tips).

A microscopic cross section of the stem reveals a column of water-conductive tissue consisting of central hydrom and outer leptom, corresponding to xylem and phloem of vascular plants respectively.

Leaves are spirally arranged at right angles to the stem; 6-8 (-12) mm long; spearhead-shaped; erect-spreading to curved and twisted when dry, spreading with recurved tips when wet; the base clasps the stem with a membranous, shiny sheath, oblong-lanceolate or elliptical, yellowish; margins with unicellular teeth from leaf base to apex; the midrib ends above the apex in a short, toothed awn; a cross section of the leaf shows the upper or dorsal surface with 20-55 vertical ridges of cells called lamellae, (4) 5-9 cells long, with apical cells distinctly enlarged and notched centrally, thick-walled and photosynthetic.

The male inflorescence consists of enlarged heads at the stem apex.

The female inflorescence frequently gives rise to a sporophyte (which produces spores in a capsule on a stalk) at the tip of the stem. Perichaetial leaves (surrounding female inflorescence) are reduced with a relatively long sheath, acuminate apex and toothed margins. Seta (capsule stalk) 5-9 cm long, wiry, yellowish to reddish brown.

Capsule 3-6 mm long, inclined to horizontal with age, almost rectangular and distinctly 4-angled, giving it a box-like appearance, brown to dark reddish brown, constricted below to a distinct hypophysis (swollen neck). Operculum (capsule lid) about 1.5 mm long, short-beaked. Peristome (a circular structure inside the mouth of the capsule) of 64 short, rounded teeth. Calyptra (capsule hood), long, densely hairy and yellowish or golden brown. Spores very small, usually released in summer.