This manual allows us to tackle the old problem of Gallicisms in the light of a great number of interesting ideas and proposals for the diachronic study of this phenomenon, in the very century of its mass introduction in the Spanish language. Indeed, this form of linguistic contamination went beyond the merely linguistic area and became a social issue in the 18th century. In the foreword to the manual there is a reflection on the translator’s bilingualism, faithfulness to the original or idioms. The purpose of the manual is to prevent translators from imitating idiomatic expressions or grammatical structures typical of the French language. For this reason, the manual includes, among others, a compared morphological study of the French and Spanish sentence and a vocabulary of French idioms. According to this article, the manual is an outstanding work of philological research involving a compilation effort unusual at that time.