The Germanic substrate hypothesis attempts to explain the purportedly distinctive nature of the Germanic languages within the context of the Indo-European languages. Based on the elements of Common Germanic vocabulary and syntax which do not seem to have cognates in other Indo-European languages, it claims that Proto-Germanic may have been either a creole or a contact language that subsumed a non-Indo-European substrate language, or a hybrid of two quite different Indo-European languages, mixing the centum and satem types.[citation needed] Which culture or cultures may have contributed the substrate material is an ongoing subject of academic debate and study.