Among the significant number of historical documents in French that discuss the principles of rhythmically uneven playing, the attention of researchers studying rhythmic alteration techniques has not yet been attracted to the testimony of Jean Baptiste Ludovico de Castillion. De Castillion (1680–1753), a Catholic priest in Ghent (later bishop in Brugge), a close friend of the famous Flemish composer, guitarist and musician of the Chapel Royal of the court in Brussels François Le Cocq (1685–1729), has received from the latter as a gift in 1729 two collections of pieces for guitar (1) composed personally by Le Cocq and (2) by various 17th century composers. De Castillion made copies of both collections, added a preface to them, and also — at the end of the manuscript — a short dictionary of the main musical terms and genre names found in the collections of Le Cocq. The manuscript of de Castillion dates back to 1730 and is currently stored in the Bibliothèque du Conservatoire Royal de Brussels under the cipher Ms.S.5615. In a brief preface, de Castillion explains the basic principles of playing on the guitar and the fundamentals of music theory. The article discusses the content of the paragraph of the preface devoted to the “movement of the eighth notes”. It is here that we see explanations concerning the specifics of using the uneven playing technique in French performance practice in the first third of the 18th century. De Castillion’s instructions concerning les notes inégales are based mainly on material from treatises by de Saint Lambert (1702) and M. P. de Montéclair (1709), but the author of the preface nevertheless had his own point of view.