In the past, social lectologists have not considered their work as contrastive linguistics. One reason is that sociolects of a language differ quantitatively; differences lie in the frequency patterns with which certain forms occur in each lect. Contrastive linguistics deals with standard or idealized languages, while sociolects are often nonstandard or socially stigmatized. Contrastive social lectology began in the 1960s to study vernacular Black English to aid in teaching standard English to minorities. Quantitative measurement of variable linguistic forms aids in examination of speech differences as related to extralinguistic factors. It is necessary to identify social and linguistic environments affecting use of variants. This article charts the occurrence of several variants by social class; analysis reveals the operation of rules governing variable appearance. Optimal use of variables is controlled by a hierarchy of constraints, but a correct theory as to how this system works has not yet been formulated. This notion of structured variability provides a model for variation in interlanguage contrastive studies. In any contrastive study it is important to examine the actual lects contacted rather than idealized norms, as illustrated in a recent study of Puerto Rican English in East Harlem. (CHK)