The Regule of Robertus de Handlo and the Summa of Johannes Hanboys are among the few major texts of medieval English music theory. The first directly influenced the latter, and both deal with unique notational practices found in English music of the fourteenth century. These two texts were edited by Edmon de Coussemaker in the nineteenth century in editions that have come to be recognized as seriously deficient. Now Peter M. Lefferts has paired them in a new critical edition that is far superior in its accuracy and scholarly underpinnings. The Regule of 1326 provides one of the two most comprehensive views of late ars antiqua notational developments. Handlo takes as his point of departure the first part, on notation, of one of those widely circulated, abbreviated versions of the teachings of Franco of Cologne that begin in most sources with the motto, "Gaudent brevitate moderni." The Summa of Hanboys, written around 1375, takes Handlo as a point of departure and incorporates an abbreviated redaction of the Regule, along with citations of other later English authorities, into an exhaustively systematic survey of ars nova forms and rests. Building on a line of development in English theory, Hanboys expanded the mensural system to a total of eight figures. For this edition, Lefferts has thoroughly reexamined, edited, and appraised the single extant source of each treatise. Full descriptions of these sources are provided and the documents are illustrated with a plate from each. Each treatise is presented in its original Latin, with a fully annotated translation on facing pages. Leffert's introduction discusses the authors, places the treatise in the context of the theoretical traditions of fourteenth-century France and England, and reviews their contents in detail. Indexes of terms, names, and subjects are included. Appendixes provide a concordance to the music examples from the Regule that recur in the Summa and transcriptions of two English motet fragments that exhibit insular notational practices discussed in the treatises. Leffert's work will be seen as a major contribution to our understanding of medieval English music.