"The present paper proposes a measure of the shape (rectangularity) of the mortality curve which is statistically orthogonal to (independent of) the level of mortality over a broad range of national life tables, both historical and contemporary.... We have proposed [using]...a ratio of survival probabilities in two broad age ranges, in middle and young adulthood.  It is suggested that this measure is an effective first approximation, which can be built on both methodologically and substantively, in order to obtain a better understanding of the social and other conditions producing rectangular and troughed mortality curves." Data for selected developed countries are used to test the model. (SUMMARY IN FRE AND ITA)