It is customary for musical historians to distinguish two Netherland schools. The first was occupied with pioneer work; its music was severe and unmelodious, simple and unpretentious when compared with that of the succeeding school, with only faint attempts to attain euphonic beauty; yet earnestness of purpose coupled with much contrapuntal science and ingenuity are everywhere in evidence. William Dufay (1400-1474) was the principal master of this school, although the mass _Ecce Ancilla_, by Antoine Busnois (1440-1492), is regarded by Naumann as ‘the most important musical historical monument up to the year 1475.’[21] In this period the several movements of the mass began to take on a certain definiteness and individuality of form corresponding to the natural subdivisions of the texts, making several movements within each movement. Likewise certain modes of treatment came to be associated with certain movements. Thus, in the _Agnus Dei_, which was divided into two parts, the composer was expected to employ the utmost resources of his contrapuntal skill; the second part was usually written in canon or in intricate fugue and frequently with a larger number of voices than in the other movements of the mass. The _Benedictus_ came to be regarded as a composition for two, three, or four solo voices, usually followed by a choral _Osanna_. And so the various movements gradually assumed quite definite outlines as to form and character, which remained in force for a century and a half.