The treatise on music of the Dominican friar Jerome of Moravia or Moray--it is uncertain whether the toponym refers to the area of central Europe or the Scottish locality--is a comprehensive study of music theory dating from the last quarter of the thirteenth century. It consists of a prologue and twenty-eight chapters devoted to the philosophy of music (chapters 1-9), its fundamentals (chapters 10-15), arithmetic and its musical applications (chapters 16 and 17), bells (chapter 18), division of the monochord (chapter 19), ecclesiastical chant (chapters 20-25), the composition and notation of polyphony (chapter 26), ancient Greek music theory (chapter 27), and the tuning and technique of stringed instruments (chapter 28). It fills its only source: Paris, Bibliothèque Nationale MS Latin 16663.  Scholars have been aware of it since 1828, when François-Louis Perne published in the Revue musicale a lengthy study (with French translation) of Jerome's chapter 28. It has proven valuable not only for its discussion of organology but for its treatments of plainchant, discant (the simultaneous combination of melodies), and musica ficta (accidentals--i.e., sharps and flats--used, but inconsistently notated, in medieval music); of the greatest interest, however, has been the evidence it presents concerning the development of rhythmic notation, which in Jerome's time was at a crucial stage, approaching a truly mensural notation in which the durations of notes would be indicated almost as precisely as in today's musical notation. Jerome's chapter 26 is a small anthology of writings (positiones) on polyphony and the notation of rhythm—including, first, an anonymous text believed to be the earliest treatment of the notational system called "modal" (a system depending on the repetition of characteristic rhythmic figures); second, a more extensive treatment of that system (ascribed to John of Garland) that provided the basis for several later treatments; third, the most highly developed treatment of the system (ascribed here to John of Burgundy, but more commonly to Franco of Cologne), the immediate predecessor of truly mensural systems; and fourth, an abbreviated summary (ascribed to Petrus Picardus) serving as an epilogue.
Jerome's treatise has been edited twice previously. Edmond de Coussemaker chose it to open his monumental Scriptorum de musica medii aevi nova series in 1864; Simon M. Cserba re-edited it in 1935.  Coussemaker, though important as a historian of music, was a lawyer by training and a magistrate by profession; he sometimes emended the text (always silently), but more often introduced errors by misconstruing abbreviations (e.g., reading partitionibus for proportionibus) and confusing one letter with another (e.g., "f" with the tall "s", thus reading figurata for signata); his publications also show a regrettably high incidence of typos. He traced the sources of more than eighty passages Jerome quoted, almost all from Boethius' De musica, but with several from the twelfth-century music theorist Johannes Cotto (olim "de Afflighem") and one from Isidore. For the third positio (Franco's) he recorded variants from concordant readings in other manuscripts. Cserba, himself a Dominican, improved considerably on Coussemaker. He provided an introduction of eighty-four pages dealing with the manuscript, the treatise and its sources, and its compiler. In the text edition he corrected many of Coussemaker's errors, though introducing others of his own. He facilitated comparison with the original or a microfilm copy by inserting into the text indications of page and column inside square brackets. He traced more than one hundred twenty quotations from earlier writers, adding to those mentioned by Coussemaker Al-Farabi, Richard of St. Victor, and Thomas Aquinas; by setting all quoted material in italics, he made it evident that the treatise is overwhelmingly compiled from prior sources. For Franco's positio he recorded variant readings from Gerbert's 1784 edition.  But he let stand many of the manifold errors in MS Latin 16663 itself; and his presentation of the text in the orthography of classical Latin, though understandable for a time when medieval Latin was seen as a degenerate form of classical Latin, does not accord with current views and standards of editing.
The new edition by Christian Meyer and Guy Lobrichon improves mightily on Cserba's. Spellings duplicate those of the manuscript (even as these vary from one instance of a word to another), so those interested in orthography can track its variations and gauge the degree of its instability in MS Latin 16663. (Abbreviations are expanded silently; punctuation follows present-day usage.) Indications of page and column, set in the margins with the point of change marked with a vertical bar in the text, do not intrude as do Cserba's; original material and quotation are set almost entirely in Roman type, so that they flow in a single stream without the disruptions of Cserba's frequent switches between Roman and italic. (In Meyer-Lobrichon, italics are reserved for the titles of musical compositions; for the ut-re-mi-fa-sol-la syllables of music theory; and for words underlined in the manuscript, to many of which glosses are attached.)
Footnotes arranged in three registers serve to identify quotations in the text; to record marginal and interlinear glosses; and to indicate corrections entered by the scribe, an early user, or the editors. Though stating that they trace only Jerome's "immediate" sources (xxi), the editors cite almost two hundred quotations drawn from twenty earlier writings--fifteen of them in editions that postdate Cserba--including two versions of the Musica enchiriadis and, in addition to Friedlein's 1867 edition of Boethius's De musica, the Glossa maior to that work recently published by Bernhard and Bower.  The editors' meticulous analysis of Jerome's sources reveals that the Boethian glosses stem predominantly from manuscripts of the French tradition, and from these they single out five specific manuscripts as its most significant representatives vis-à-vis Jerome (xviii); that some quotations in the prologue and chapter 1, and virtually the entire content of chapters 4-8 (including quotations from Boethius, Al-Farabi, Richard of St. Victor, and Isidore), draw on the Speculum doctrinale of the Dominican Vincent of Beauvais; and that more than three hundred lines (21.78-113 and 22.5-288; the numbers refer to chapters and lines) are taken from the Tonarius fratrum praedicatorum (147-56).  These provide clear evidence of the treatise's Dominican complexion. (Cserba had already identified some chants in Chapter 22 as occurring in the Antiphonarium O. P.) In 24.103-46, the editors identify cited chants by liturgical function and location (164-65), and in chapter 26 they indicate sources of some of the polyphonic compositions quoted.
The editors emend the text of MS Latin 16663 through comparison with the texts of Jerome's many sources. For instance, in 7.5-7 and 7.19-21, drawn from Richard of St. Victor, they are able--by restoring words omitted evidently through saut du même au même--to improve passages expounding the classification of the Boethian concepts musica mundana and musica instrumentalis:
"Mundana alia in elementis,
(Note, though, that the et after flatu should be ut, as it is in Richard's text.)  In one such case (26.1806-37) the text restored by the editors fills thirty-two lines (240-41).
Not all attempts at emendation are successful. For instance, where in 24.7-8 a clause lacks a verb, the editors supply one as follows--
"Sicut enim non omnium ora eosdem cibos
--borrowing the verb from Jerome's source, Johannes Cotto's Musica. But what Johannes had is
"Sicut enim non omnium ora eodem cibo capiuntur..." 
The editors seem to have overlooked that where Johannes had eodem cibo, Jerome has eosdem cibos; this accusative requires a verb that is active, not passive, of which it will be the object.
One might ask whether all the editors' emendations are warranted. For instance, they emend a passage that Jerome took from Thomas Aquinas's commentary on Aristotle's De caelo et mundo as follows:
"...primo [pro]ponit quod intendit, secundo inducit racionem aliter senciencium, tercio ostendit quomodo dubitacioni satisfacere nituntur" (23).
The editors' square brackets indicate deletion of the prefix pro, bringing Jerome's text into agreement with Thomas's (they cite the editio leonina). But ponere and proponere have broad arrays of meanings, overlapping to a degree, with both including "to put forward for consideration or examination." Proponit, then, would seem here as suitable a choice as ponit, and how do we know that this is not what Jerome's exemplar had? Why, indeed, emend a thirteenth-century text to make it conform to a nineteenth-century edition of another text? Of course, the editors could not conceivably have consulted countless medieval copies of Thomas's text; nonetheless, I'd have preferred a less intrusive way of indicating how Jerome's text differs from the standard edition of Thomas's.
Though the editors' introduction is admirable in providing a concise description of the manuscript and an account of its history, as well as information about the author and date of the treatise, several items normally--and appropriately--found in critical editions are lacking. Because of the treatise's importance for the topics of mode, discant, and especially mensuration, discussions of Jerome's treatments of them would have been welcome as part of the Introduction. Indexes of subjects and of Latin terms would have greatly enhanced the volume's utility for scholars. (The Index fontium can serve as an index of names.) The bibliography is selective rather than exhaustive; readers can augment it through consultation of Die Musik in Geschichte und Gegenwart, Grove Music Online, and The Theory of Music, Volume 6. 
Despite these omissions, this meticulously prepared edition of an important treatise improves significantly on its predecessors. It will now serve as the standard edition of Jerome's treatise, and should find a place in all research libraries. It will benefit scholars interested in medieval music, music theory, codicology, and intellectual history.
1. A color scan of the manuscript is available through Gallica, the web site of the BNF: http://gallica.bnf.fr/ark:/12148/btv1b8432480x.r=Latin+16663.langEN
2. Edmond de Coussemaker (ed.), Scriptorum de musica medii aevi nova series, 4 vols. (Paris: Durand, 1864-1876; reprint, Hildesheim: Olms, 1963), vol. 1, 1-154; Hieronymus de Moravia, O. P., Tractatus de musica, ed. Simon M. Cserba, O. P. (Freiburger Studien zur Musikwissenschaft 2; Regensburg: Pustet, 1935).
3. Martin Gerbert (ed.), Scriptores ecclesiastici de musica sacra potissimum, 3 vols. (St. Blaise: Typis San-Blasianis, 1784; repr. Hildesheim: Olms, 1963), vol. 3, 1-16.
4. Michael Bernhard and Calvin M. Bower (eds.), Glossa maior in institutionem musicam Boethii, 4 vols. (Bayerische Akademie der Wissenschaften, Veröffentlichungen der Musikhistorischen Kommission 9-12; Munich: Verlag der Bayerischen Akademie der Wissenschaften, C. H. Beck, 1993-2011).
5. Edited in Christian Meyer, "Le Tonaire des Frères Prêcheurs," Archivum Fratrum Praedicatorum 72 (2006): 117-156, 138-145.
6. Richard de Saint-Victor, Liber exceptionum: Texte critique avec introduction, notes et tables, ed. Jean Chatillon (Textes philosophiques du moyen-âge 5; Paris: Vrin, 1958), 108.
7. Joseph Smits van Waesberghe (ed.), Johannis Affligemensis De musica cum tonario (Corpus Scriptorum de Musica 1; Rome: American Institute of Musicology, 1950), 109.
8. Christian Meyer, "Hieronymus de Moravia," Die Musik in Geschichte und Gegenwart, 2nd ed., ed. Ludwig Finscher, Personenteil, vol. 8 (Cassel and New York: Bärenreiter, 2002), 1519-1521; Frederick Hammond and Edward H. Roesner, "Hieronymus de Moravia," Grove Music Online; Christian Meyer et al. (eds.), The Theory of Music, Volume 6: Manuscripts from the Carolingian Era up to c. 1500--Addenda, Corrigenda, Descriptive Catalogue (Répertoire International des Sources Musicales, B III, vol. 6; Munich: Henle, 2003), 226-7.