This paper presents key aspects of the data, methods and uses of the From Inglis to Scots (FITS) Corpus (Alcorn et al., 2021–), complementing Kopaczyk (2018) and focusing on the diachronic dimension of the resource. The corpus reconstructs sound values for individual Older Scots morphological root elements of Germanic origin as attested in the documentary record in Linguistic Atlas of Older Scots (LAOS). This is done by triangulating between the attested spelling, the sound values proposed in the literature for their etymological sources (dialects of Old English, Old Norse and Middle Dutch), and a series of plausible sound changes leading from the latter to the former (the Corpus of Changes). The challenges and possibilities of this approach are highlighted throughout, focusing on the diachronic mapping of Older Scots sounds to their origins and the intervening changes. An overview of the corpus’ capabilities is provided in tandem with its limitations.