Plagiotheciaceae M.Fleisch.

The family is usually considered by modern authors to be monogeneric (Ireland 1986), although Brotherus (1925) included other genera. A detailed description of the family (and the genus Plagiothecium) is provided by Buck & Ireland (1985), who characterised the family as "pleurocarpous mosses that are mostly complanate-foliate with stems that have an outer row of thin-walled cells, leaves that are decurrent and often asymmetric with costae short and double, leaf cells that are smooth, ± linear, and heavily chlorophyllose, and differentiated alar cells that are confined to the basal angles."

The family is retained in the order Hypnales by Goffinet et al. (2009), who also included the genus Struckia Müll.Hal. This small Asian genus has been placed traditionally in Sematophyllaceae; it is not considered here. Crum & Anderson (1981) hesitantly included the poorly understood tropical genus Stereophyllum Mitt. in the Plagiotheciaceae; it is not considered here either.