The present publication is intended to fill, in some measure, the most serious gap in our knowledge of the history of polyphonic music, that is, the development of French music between Machaut and Dufay. - Preface. The present publication is devoted to the first of these two schools, formed by the immediate successors of Machaut. The total repertory contained in the sources of this period consists of liturgical pieces (mass items, etc.), motets, and secular compositions, with the last category far outnumbering the two others. For instance, the above-mentioned three main sources contain approximately a dozen sacred compositions, about 15 motets, and over 200 secular pieces, mostly with French, but occasionally with Italian and Latin texts. In the following study only the French secular compositions, exclusive of those by Machaut, are considered. More than half of these are attributed to composers. The number of these composers is surprisingly great, and a complete list would include more than forty names. Practically all the compositions of our repertory belong to one of the three traditional formes fixes of French medieval poetry and music, that is, the ballade, the virelai, and the rondeau. The repertory of our sources can be divided into three stylistic groups, A, B, C, which can reasonably be assumed to represent three phases of a continuous development. These phases, for which the names "Machaut Style," "Manneristic Style," and "Modern Style" will be used, may be said to extend approximately from 1350 to 1370, from 1370 to 1390, and from 1390 to 1400, naturally more or less overlapping. - Introduction.