This book investigates the cause and consequences of linguistic change through an analysis of the fate of øs! in the Romance languages. A comparison of the mechanism of the weakening of øs! in modern Spanish with historical changes in Italian and French shows that the process is set in motion by a sound change which leads to a reduction of articulatory effort. Given the role of øs! as a number and person morpheme in these languages, its weakening leads inevitably to morphological restructuring which in no case results in an increase in the effort saved in the production of the message. The solutions adopted in the history of French and Italian provide a basis for predicting the ultimate and divergent outcomes of the ongoing processes in Spanish dialects.