Morphosyntactic isoglosses in Indo-European: An introduction

The last decades are marked with an increasing interest towards the study of isoglosses shared by some branches of the Indo-European language family. As is well-known, next to well-established branches such as Germanic, Greek or Indo-Iranian, there are larger subdivisions within Indo-European, grouping together several branches, in accordance with a number of features, traditionally called isoglosses. Such features are shared by more than one group, or by several languages which do not belong to the same group (branch-crossing isoglosses). Such isoglosses have always been at the focus of vivid debates in Indo-European scholarship, giving rise to numerous hypotheses on early splits within Proto-Indo-European or, on the contrary, later contacts among historically attested languages. A systematic research of these issues still remains a desideratum. Next to a few notorious isoglosses, almost exclusively limited to the phonological level, such as the kentum/satəm division,1 or the ‘ruki’ division (retraction of the sibilant s), which have been known for about a century, there are a few less studied morpho-syntactic features, often of a much vaguer nature, that equally group together a number of branches and/or languages. One