Abstract
1. Previous papers on the subject of the os trigonum are reviewed.
2. Evidence is produced to show that the posterior part of the talus normally develops from separate centres.
3. An explanation is given for the presence in adults of separate ossicles (the so-called os trigonum) in one or both ankles and for the variety of shapes adopted by them.
4. Contrary to opinions expressed by previous writers, these ossicles may give rise to symptoms.