As a language family with some 3000 years of documentation, and spread over a vast territory, the Iranian branch of Indo-European is a fortunate case for testing hypotheses and methods of relations among related languages. Conversely, while there is a large number of Ir. languages, Persian is the only member of the family that is attested in Old, Middle and New Iranian (one of the IE languages with the longest attested history) while all others are only attested in one of the three periods. In addition to the difficulties involved here, there is the interesting question of calculating the contribution of the languages that died out. Another perspective is provided by the large bodies of new data which have come to light both of contemporary and of historical Ir. languages. The latest arrivals include Bactrian, the language of the Kushan empire. In spite of its historical importance, it was known until 1990 only from coins and a few stone inscriptions (1st-3rdc. AD) that proved difficult to read and to understand. In the intervening years, a corpus of some 150 manuscripts (letters, contracts etc., 4th-9thc.) has come to light and is available for study now. So far as research of the relations among Ir. languages is concerned, the new data have not been integrated yet. Also, the methodology has remained essentially unchanged, and those works that do address the issue make use of essentially the same set of isoglosses as 100 years ago. These isoglosses are not only outdated, but also problematic from the outset because they were established for other purposes (e.g. the differentiation of various Ir. “dialects” present in the Manichean manuscripts found in Chinese Turkestan at the beginning of the 20th c.). A fresh look, and a new collection of features, is thus urgently needed. Taking the morphological innovations of Bactrian as an example, unexplored for purposes of language grouping so far, a complex pattern of their being shared by other Ir. languages emerges. Noteworthily, the features cross the dichotomy of Eastern and Western Iranian, which has been the very basis of Ir. dialectology, and reveals a particularly close relation with Parthian. Likewise remarkably, the picture shown by these features of Bactrian is not without similarities to the findings on isoglosses of other language families, and is in fact quite similar e.g. to the result obtained for of Oceanic languages by Alexandre François, promising fruitful perspectives for an exchange of research results on various language families. At the same time, it seems to me that one does not necessarily need to stop here: Ir. data allow for study of the diachronic perspective, which in turn permits evaluations that are necessarily impossible in the case of many other language families. The paper will thus “attack” the picture presented by the diagram of morphological innovations of Bactrian shared with other Ir. languages, and discuss how it changes when adding aspects such as archaism vs. innovation (NB that the two sides of an isogloss i.e. the absence vs. presence of a feature may carry entirely different implications for language groupings), relative chronology, and the possible impact of neighbouring languages within and outside the group (which in the case of Iranian are mostly quite well documented).