This book examines the relationship between political identity and variation from a diachronic perspective, and how vernacular features could index political and ideological affiliations. Specifically, it explores the use of written Scots in the correspondence of Scottish politicians active during the Union of the Parliaments debates at the turn of the eighteenth century. Drawing from the frameworks of First, Second and Third Wave perspectives on variation, and combining macro-social statistical modelling with microsocial analysis, broad socio-political factors are empirically investigated alongside plausible stylistic intentions in conditioning observed linguistic behaviour. Detailing the process of building a corpus, identifying relevant Scots features, and presenting analyses across time, sociolinguistic factors, and individual recipients, this volume provides the first in-depth quantitative and sociolinguistic examination of early modern Scots and its expanding indexical roles.