In spite of the vast amount of research on mood in Romance languages, certain fundamental issues are clearly underrepresented, particularly in the field of diachronic linguistics. With this in mind, the primary goal of this dissertation is to provide a comprehensive explanation for developments in Romance mood distribution. Unlike the majority of existing research, this approach does not analyze mood in isolation, nor does it look outward for language-external explanations. Instead, changes in mood usage are related to major typological developments via several interconnected analyses which rely heavily on data from Latin and medieval Spanish and French.

This investigation, which takes as its starting point the well-attested typological shift from OV to VO word order, addresses four major issues. The first of these is branching congruency, whereby post-posed subordinate clauses are more closely associated with explicit subordinating conjunctions. This hypothesis is tested via a quantitative analysis of Latin data, in order to establish a link between conjunctions and VO word order. The development of these subordinating elements is then analyzed within the grammaticalization framework, which provides insight into the nature of specific Latin and Romance forms, in addition to demonstrating the usefulness of certain theoretical notions. The outcome of this process is a highly generalized Romance subordinator, which is argued to have undergone partial synthesis with the subjunctive, evidenced by an increase in both obligatoriness and contiguity.

Finally, these cumulative changes in the linguistic system are shown to have had substantial destabilizing effects on the existing subjunctive/indicative contrast. The significance of this claim is that, already in Latin, mood selection is characterized by a loss of motivation and an increase in automaticity. As a result, subsequent changes in mood distribution in Romance languages are not viewed merely as reductive phenomena, but rather as signs of the refunctionalization of a destabilized, yet viable, paradigm.