Socio-historical linguistics (or historical sociolinguistics) is a relatively young dicipline that still needs refinements in the definiton of its scope, methods and theoretical foundations. Its relationship to historical linguistics also deserves further thinking.
The texts produced in the numerous centres of writing of the Italian peninsula in the Early Middle Ages make a good testing ground to assess methods and assumptions of the historical study of languages in their social and cultural contexts and more generally to reconsider old problems of the historical study of languages. In particular they have the potential for the exploration of new possibilities to understand the changes of Latin into the Romance vernaculars.
With its multiethnic population that included peoples coming from various parts of the Eastern Mediterranean (especially in the South) and from Continental Europe (mainly, though not exclusively, in the North) and its rich diversity of linguistic and cultural contexts, the Italian peninsula of the so-called „dark centuries“ (i.e. those from fifth to tenth CE) is an ideal cultural and linguistic area to face the tasks mentioned above.
Heir to the Roman linguistic and cultural legacy in its own distinct way, perhaps more than other areas in Europe post-Roman Italy was politically and culturally torn between the East and the West. It was a fronteer between politically and culturally diverse territories with a complex chronology and geographical distribution: the Byzantine domains (Ravenna and the Exarchate intermittently until the last decades of eighth century, the dukedoms of Gaeta, Naples and Amalfi until the tenth / eleventh centuries, Sicily until the Arab invasion in the ninth century) on the one hand, and the Romano-Germanic kingdoms on the other (the Goths from the end of fifth to the middle of sixth century, the Langobards from the end of sixth to the end of eighth century, the Franks from the end of the eigth century onwards). It was a crossroad not only of cultures but also of languages: Latin, Greek and those of the Germanic group, just to mention the major languages in contact, and also Armenian, the varous Semitic and Slavonic languages spoken by the numerous ethnic minorities or by the larger groups of population as was the case of Arabic and Hebrew in Sicily under the Arabsʹ rule.
The wealth of linguistic and cultural contexts of the Italian peninsula is reflected in the great number of texts of various types written in Italy. Of special interest to the linguist are those of legal nature which through the veil of formulaic structures bear witness to real usages of the everyday language.
Notaries and scribes, the professionals of language and writing who abounded in many areas of the peninsula emerge as key figures for the socio-historical study of Italy of the Early Middle Ages. Through the texts they wrote we can catch a glimpse of the areal, social and stylistic variation and in some cases infer the changes of Late Latin, its phenomena of contact with Greek and the Germanic languages and the early emergence of the Romance vernaculars.

Basic introductory references to theories and methods
Hernández-Campoy, Juan M. & Conde-Silvestre J. Camilo, 2012 (eds.), The Handbook of Historical Sociolinguistics, Chichester (UK), Wiley-Balckwell.
Romaine, Suzanne,1982, Socio-historical linguistics its status and methodology, Cambridge, Cambridge University Press.
Sornicola, Rosanna, Le regole del gioco in sociolinguistica storica. Alcune riflessioni su metodi, possibilità e limiti di una disciplina controversa, in Carlo Consani, Carmela Perta (a cura di), Dinamiche del multilinguismo. Aspetti teorico-applicativi fra oralità e scrittura, Quaderni di AION-L, n.s. 6, Napoli, UniorPress, 41-90.