Penrose and Smith (1966) have reviewed the literature on Down's syndrome in great detail, and this has been followed by an important recent review by Richards (1973). In Chapters 10 and 11 of Penrose and Smith's book they discuss the remarkable frequency distribution of the ages of mothers of patients, compared with that of the general population at the corresponding place and time, and they summarize the large number of studies made on this subject. The mean age of the mothers is shifted upwards by amounts which vary in different countries from about 6 to 8 years. The remarkable feature, however, is that there appear to be two bumps in the curve. These are usually (but not always) not large enough to make the curve bimodal, and J. B. S. Haldane therefore coined the term ‘bitangentiality’ for this phenomenon, which appears in most published studies and in the group of all sample cases (9,441) given by Penrose and Smith, Fig. 76. Collmann and Stoller (1962) make a complete survey of all mongol births in Victoria, Australia, from 1942 to 1957 and here there is a distinct bimodality.