This study outlines a methodological framework for reconstructing linguistic history within a dialect continuum and applies this methodology to an under-described, controversial, and complex subgroup of New Indo-Aryan (NIA)—the Kamta, Rajbanshi and Northern Deshi Bangla lects (KRNB). ¶ Dialect continua are characterised by non-discrete boundaries between speech communities, and as a result previously divergent lects may undergo common innovations; the result is the familiar picture of overlapping dialectological isoglosses. The sequencing of these innovations and the historical relations between the lects involved are often highly ambiguous. Given the right sociohistorical conditions, a widespread innovation may be more recent than a localised innovation—the very opposite sequencing to that implied by the splits in a family tree. ¶ Not surprisingly, discrete application to the NIA continuum of traditional methodologies—including the Comparative Method, etymological reconstruction and dialect geography—has yielded unsatisfactory and at times chronologically distorted results. Historical studies, therefore, have chosen between: (a) only studying the histories of NIA lects with written records; (b) reconstructing using the chronology suggested by the shape of a family tree; or (c) settling for a ‘flat’, non-historical account of dialect geography. ¶ Under the approach developed here, the strengths of each of these traditional methods are synthesised within an overarching framework provided by a sociohistorical theory of language change. This synthesis enables the linguistic history of the KRNB lects to be reconstructed with some detail from the proto-Kamta stage (1250-1550 AD) up to the present day. Innovations are sequenced based on three types of criteria: linguistic, textual and sociohistorical. The old Kamta stage, and its relation to old Bangla and Asamiya, is reconstructed based on linguistic Propagation Events and Speech Community Events—two concepts central to the methodology. The old Kamta speech community and its language became divided into western, central and eastern subsections during the middle KRNB period (1550-1787 AD, dates assigned by attested sociohistorical events). During the same period, KRNB lects also underwent partial reintegration with NIA lects further afield by means of more widely propagated changes. This trend of differentiation at a local level, concurrent with reintegration at a wider level, also characterises the modern KRNB period from 1787 AD to the present. ¶ This account of KRNB linguistic history is based on a rigorous reconstruction of changes in phonology and morphology. The result is not only a reconstruction of historical changes, but of the proto-Kamta phoneme inventory, hundreds of words of vocabulary, and specific areas of nominal and verbal morphology. The reconstruction is based on data collected in the field for the purposes of this study. Phonological reconstruction has made use of the WordCorr software program, and the reconstructed vocabulary is presented in a comparative wordlist in an appendix. ¶ The methodology developed and applied in this study has been found highly successful; though naturally not without its own limitations. This study has significance for its contribution both to the methodology of historical linguistic reconstruction and to the light shed on the linguistic prehistory of KRNB.