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In page Slavic settlement of the Eastern Alps:

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The prevailing view on the Slavic settlement of the Eastern Alps is based mostly on evidence deduced from archeological remains (many of which have been discovered due to the extensive highway constructions in post-1991 Slovenia),[1] ethnographic traces (patterns of rural settlement and land cultivation), as well as on the ascertainments of historical linguistics (including toponymy). Besides, it is fully confirmed by the relatively few available contemporary mentionings and early historical sources (such as Historia Langobardorum by Paulus Diaconus[2] or letters from Pope Gregory I[3]). Another important evidence of Slavic advances is the progressive decline of ancient Christian dioceses in the respective areas.