We may [...] state more precisely that under-differentiation of a distinctive feature may affect all, or only some, of the phonemes of the 'S'-language which are characterized by that feature.
- Weinreich (1957), a pag.6

Whenever a bilingual classifies a feature of sound as distinctive which in the original 'S'-language is redundant, we may speak of OVER-DIFFERENTIATION. In the opposite case — as when a speaker of 'P'-Spanish analyzes the fricativity of an intervocalic English [ð] as redundant — we have UNDER-DIFFERENTIATION. The advance represented by this statement compared to that in 'LiC' [Weinreich, U., 1967, Languages in Contact. Findings and Problems, Columbia University, Mouton & Co., p. 18] lies in the realizations that (a) over- and under-differentiation are matters of distinctive features rather than of phonemes [...] (b) over- and under-differentiation are matters of analysis rather than rendition; even under-differentiation may lead to impeccable renditions.
- Weinreich (1957), a pag.5-6