The largest class of the division Bryophyta, the true mosses. Members of the class are best characterized by operculate capsules and a peristome that aids in the dispersal of spores, and are generally perennial. The class consists of about 14,000 species distributed in six subclasses based primarily on the structure and developmental history of the sporophyte and especially the peristome. The orders and families are likewise based primarily on stable sporophytic details, whereas genera and species are most often differentiated in terms of gametophytic features.