This study assesses the relevance of the methods of calculating mobility rates as a means describing and interpreting the nature and degree of structured social inequalities in a given system of social stratification. It shows how these calculations are made and what kind of interpretations can be warranted, on the basis of these rates, about the nature and degree of social closure and rigidity of a given stratification system. The basic argument of the paper is threefold, each part being presented and discussed on the basis of concrete examples. Firstly, it is argued that, mobility rates alone are not sufficient to warrant any interpretation about the degree of social closure or fluidity. Secondly, absolute and relative mobility rates capture different aspects of inequalities and thus they serve to different ends. Finally, when there are legal, political and institutional arrangements that overtly or covertly discriminate individuals and groups and/or put restrictions on their routes of mobility, it would be more appropriate, on the basis of the same rates, to assess the stratification system not in terms of openness but in terms of its rigidity and flexibility.