Ischial callosities are specialised regions of skin and subdermal tissue in the form of fibro‐fatty cushions with a tough, non‐slip surface, firmly bound to the underlying ischial tuberosity. They occur in primates of the families Cercopithecidae and Hylobatidae. Ischial callosity usage helps animals to adopt stable sitting postures on the tops of branches, particularly during feeding, resting and sleeping. Callosity‐like areas in pongids are mainly epidermal specialisations and differ in their development and usage from true callosities. They are similar to the para‐callosity skin regions in some ground living cercopithecids. The ischial tuberosity in animals with true callosities has a flat surface which flares out from the lower end of the ischial body and which is devoid of muscle attachments. In animals without callosities the ischial tuberosity is not flared, has a curved surface, and acts as a site for muscle attachment. The ischial tuberosity is relatively broader in animals with callosities than in animals without callosities, although the absolute size of the specimens concerned must be taken into account. The combination of qualitative and quantitative features of fossil ischial tuberosities enables the presence or absence of callosities in these forms to be deduced. The recognition of these features in fossil material might be of use in the elucidation of the evolution of one type of primate arboreal adaptation.