The genus Didymodon as treated here is difficult to characterize because it is so variable in nearly all features. Saito (1975) considered the genus distinct on the basis of derived traits such as brownish basal cells of the axillary hairs, short cells on the upper ventral (adaxial) surface of the costa, and well defined laminal cells. Steere (1938, 1947) and Zander (1994a) advocated subdividing Didymodon, with the latter treatment outlining four evolutionary lines that corresponded to the sections Didymodon, Asteriscium (C. Müll.) Zand., Fallaces Steere, and Vineales (Steere) Zand. Section Asteriscium is the most distinctive of these Didymodon sections; its members have a stem hyalodermis, a single (dorsal) stereid band in the costa, and greatly enlarged, papillose cells on the upper ventral surface of the costa. They are treated here as the segregated genera Husnotiella and Trichostomopsis. In a more recent evolutionary analysis of the genus (Zander 1998) only two sections were recognized: sect. Didymodon (including sections Asteriscium and Vineales) and sect. Fallaces.

Didymodon is close to the genus Barbula. The two genera are sometimes considered synonymous (Smith 1978, Nyholm 1990, Frey & Kürschner 1991, Abramov & Volkova 1998). It is impossible to avoid the terms “usually, mostly, occasionally” when comparing the two genera, and in fact the only absolute feature separating them is an axillary hair feature: Barbula has entirely hyaline axillary hairs while Didymodon has axillary hairs with one or more brown basal cell(s). In general terms Didymodon differs from Barbula in leaf shape (lanceolate to long-lanceolate vs triangular to lingulate), relative basal and ventral leaf cell differentiation (weakly vs strongly), shape of the upper ventral costal cells (quadrate vs elongate), leaf papillae (absent to simple vs multiplex) and peristome teeth (short, erect to weakly twisted vs long and strongly twisted).