My dissertation offers a case study on the relationship between language and power during a period when a series of institutions, ideas, and practices surfaced that transformed European society and significantly altered the manner in which rulers governed. The growth of bureaucracies within increasingly centralized monarchies, a gradual process of laicization, and the rapid spread of vernacular writing represent three of these major changes as well as the primary focus of this survey. In it, I analyze systematically what drove the thirteenth-century kings of the Crown of Aragon to begin writing in their realms' spoken vernaculars (Catalan and Aragonese), what these motivations reveal about contemporary mentalities and language ideologies, and how codeswitching (shifting from Latin to the romance and back) figured into the crown's overall governing practices. Using a multi-disciplinary approach that combines historical methodologies with theoretical frameworks from linguistic anthropology, sociolinguistics, and literary criticism, I conclude that the kings appropriated codeswitching strategies as diplomatic instruments in the administration of their realms. The practical impulse to reach a larger, Latin-illiterate audience influenced all vernacular writing at some level during the Middle Ages, including within the royal chancery. Yet, in the vast majority of cases the kings appropriated vernacular writing strategically as a symbolic instrument. In these instances, the switch from Latin to a vernacular language alone signaled a change in tone, usually denoting displeasure or urgency. Employing language as a political tool betrayed the kings' weakness rather than their strength. Like their contemporaries throughout Western Europe, the kings of the Crown of Aragon were in the initial stages of consolidating and expanding their power over a recalcitrant nobility through a combination of violence, better organized bureaucracies, and the support of urban elites. Language choice between writing in Latin and the romance offered the kings an effective and simple means to communicate more forcefully with their subjects, and, in doing so, better exert their relatively fragile authority.