Koineization – the development of a new, mixed variety following dialect contact – has well-documented outcomes. However, there have been few studies of the phenomenon actually in progress. This article describes the development of a new variety in the English New Town of Milton Keynes, designated in 1967. The article is structured around eight “principles” that relate the process of koineization to its outcomes. Recordings were made of 48 Milton Keynes-born children in three age groups (4, 8, and 12), the principal caregiver of each child, and several elderly locally born residents. Quantitative analysis of ten phonetic variables suggests that substantial but not complete focusing occurs in the child generation. The lack of linguistic continuity in the New Town is demonstrated, and the time scale of koineization there is discussed. Finally, it is shown that demography and the social-network characteristics of individuals are crucial to the outcomes of koineization.