The Traité de modulations written by French guitarist and composer Adolphe Le Dhuy first appeared as an appendix to his Études caractéristiques pour la guitare Op. 21. It is a text that has remained overshadowed until now, but that can offer an interesting insight into the practice of harmony in the nineteenth century, and join the meager corpus of guitar methods and treatises of the time. While Le Dhuy largely draws from Dionisio Aguado's method (with the appendix by François de Fossa), there is no shortage of his original contributions. Various chords, cadences, and modulations — written and fingered in five positions — can still today offer insights into the study of harmony and improvisation on the guitar fretboard as well as help us understand how this study was included in the compositional and performance practices of the time. Indeed, this material was conceived by Le Dhuy as preparatory to the practice of preluding and modulating, that is, the extemporaneous composition of a prelude linking the end of one piece in one key to the beginning of the next in another key. Furthermore, as he writes in the treatise, "[this material] can serve to vary the endings of pieces, which almost always resemble one another." It is indicative, from this point of view, that the Traité de modulations was published as an integral part of the Études Caractéristiques Op. 21, not as a stand-alone text. In addition to the original French text, this modern edition published by Saving Zerboni is accompanied by both an Italian and English translation.