DIACHRONIC DIALECTOLOGY deals (a) with DIVERGENCE, i.e it studies the growth of partial differences at the expense of similarities and possibly reconstructs earlier stages of greater similarity (traditionally, comparative linguistics); (b) with CONVERGENCE, i.e. it studies partial similarities increasing at the expense of differences (traditionally, substratum and adstratum studies, "bilingual dialectology", and the like).
- Weinreich (1954), a pag.395