This book studies the differences of pronunciation and grammar that exist within the Spanish-speaking world, and traces their origins in the frequent mixing of dialects in Spanish-speaking communities from the Middle Ages to the present day. It emphasizes the subtlety and seamlessness of language variation, both geographical and social, and shows how the constant process of mixing has rendered Spanish particularly subject to leveling of its linguistic irregularities and to simplification of its structures, both in Europe and later in the Americas.