Contents -- Introduction -- Inflectional complexity of nouns, adjectives and articles in closely related (non-)isolated varieties -- On the origins of complexity: evidence from Germanic -- An information-theoretic approach to assess linguistic complexity -- On complexity of interrogative syntax in Northern Italian dialects -- Complex edges, transparent frontiers: grammatical complexity and language spreads -- A true split? Typological and sociolinguistic considerations on contact intensity effects -- The sociolinguistics of non-equicomplexity.