This paper discusses some problems relating to the study and the utilization of discontinuous traits in anthropology. The importance of sex, age and laterality in the manifestation of these traits is discussed on the basis of investigations on a numerous skeletal sample of Siena (Tuscany, Italy) and on data from literature. It is pointed out that sides and sexes should be considered separately. The age of adult subjects seems to be insignificant as the incidence of these traits is concerned.