Indo-Slavic lexical isoglosses and the prehistoric dispersal of Indo-Iranian
Shared lexical innovations between Indo-Iranian and Balto-Slavic reveal a period of shared development — the Indo-Slavic linkage — meaning Indo-Iranian did not spread straight eastward from the Core Indo-European homeland; it moved into northeast Europe during the 3rd millennium BCE and only then spread east toward the Urals.
— In our words · Source: Indo-Slavic lexical isoglosses and the prehistoric dispersal of Indo-Iranian (A.I. Palmér)