Dual number marked on personal pronouns and verbal agreement suffixes disappeared in the majority of Slavic languages except for three - Slovenian, Upper Sorbian, and Lower Sorbian. Previous studies do not provide a principled account why (i) the monomorphemic dual was reanalyzed as bimorphemic in Slovenian, Upper, and Lower Sorbian, and why (ii) it was replaced by the plural in the majority of Slavic languages including Russian and Kashubian. In this dissertation, I investigate diachronic changes in the morphosyntactic category of the Slavic dual which occurred in the 11th-15th centuries. In this dissertation, I present new diachronic data obtained through digital corpora of Old Slavic manuscripts. Within the framework of Distributed Morphology, I propose a new principle of Morphosyntactic Feature Economy which explains these two different patterns of diachronic change in the Slavic dual pronouns and verbal agreement. I argue that the principle of Morphosyntactic Feature Economy is one of the factors which drives diachronic change in the category of number in Slavic languages. Morphosyntactic Feature Economy is a principle of efficient computation. It plays an essential role in restructuring of the morphosyntactic category of the Slavic dual. Morphosyntactic restructuring of the Slavic dual, triggered by its morphosyntactic and semantic markedness, results in a morphosyntactically 'simpler' category of number. As a consequence of the application of the principle of Morphosyntactic Feature Economy, the language learner acquires a featurally restructuctured category of number, which is simpler and more computationally efficient.

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