In the first period of polyphony the singers had followed the inspiration of the moment and certain general rules of intervallic movement in improvising their discant to the Gregorian chant. In the fourteenth century these unsystematic efforts gradually gave way to the definite writing of all the parts to be sung. In the fifteenth century the Netherlanders began systematically to develop and perfect the forms crudely outlined by their predecessors in the fields of both church and secular music. The forms of church choral music that held their chief attention were masses, motets, psalms, and hymns. Among the secular forms we find _chansons_ and madrigals. Of all these the mass, with its separate parts, was destined to become the form on which the composer expended his greatest care and skill and through which he sought to express his noblest thoughts. It was to the Netherland period and to the Roman Church composers thereafter what the sonata and the symphony were to the composers of the nineteenth century and the decades just preceding. In such reverence and respect was this form held that in the preface of a mass published in 1539 by Grapheus in Nuremberg it could be confidently asserted, ‘he who is not acquainted with the masses of the old masters is ignorant of true music.’ The great importance attached to the mass by composers was inevitable from its commanding position in the church service. At this point it may be opportune to discuss some of the essential features of the mass from the standpoint of the liturgy.