Until now the concept of idiosyncrasy has been hardly associated with syntax or syntactic relations. Contributions of Indo-European linguistics show that it must be examined if there are indeed idiosyncratic syntactic phenomena. The data comes from Hittite, Vedic, Sanskrit, Greek, Latin, Venetic, Runic Norse, Welsh, Early New High German. Research questions are: Are idiosyncrasies in syntax actually irregularities or must they be explained by not stated rules? How do idiosyncrasies emerge historically? How are they being dealt with?