Complex adverbials built with the pattern ‘Preposition + Adjective (PA)’ such as Portuguese em especial ‘especially’ or Romanian de sigur ‘for sure’ display a remarkable frequency and pan-Romance consistency. Nevertheless, this pattern has been largely neglected by research. The ‘Third Way’ project intends to fill this gap, arguing that PAs are a relevant third way of constructing adverbials, aside with short adverbs and adverbs in - mente . It subjects prepositional adverbials to a historico-varietal analysis in a comparative pan-Romance approach to trace their trajectory from Latin up to now. This article presents an enquiry design for field research that allows examining the adverbials’ current usage and ongoing change in substandard varieties of all major Romance languages. The results are meant to help reconstruct the diachrony of PAs in Romance, especially in view of the spoken vernacular tradition.