Although the word ‘mass’[16] is, strictly speaking, applicable only to the eucharistic service in its entirety, it has been used from the early centuries of Roman Church history to designate certain portions of the liturgy to which unusually solemn and impressive music has been set. With the growth of counterpoint the opportunities for increasing the impressiveness and elaborateness of these settings were obviously multiplied. The parts of the service which were thus subject to special musical elaboration were the _Kyrie_, the _Gloria_, the _Credo_, the _Sanctus_, the _Benedictus_, and the _Agnus Dei_. These six movements together comprise what was known as the ‘mass,’ and they still constitute, with slight variations, the essential portions in all musical masses, whether written for church or concert performance. During the period under consideration it was an almost universal custom to have one subject (_cantus firmus_) do service for all the movements of a mass, which accordingly took its name from this subject. These subjects, particularly in the earlier periods of polyphonic music, were plain-song melodies, whence we have such names for masses as _Missa Iste confessor_, _Missa Tu es Petrus_, and _Missa Veni sponsa Christi_. But, as has already been mentioned, sacred melodies were not the only ones chosen. Composers frequently invaded the domain of popular song for subjects for their masses. Such ardent love-songs as _Adieu, mes amours_ (‘Farewell, my love’) and _Baisez-moi_ (‘Kiss me’) seem strangely out of place in such surroundings, but these and similar names appear in the titles of many a mass of this period. The most famous of all the popular songs thus used was the old French love-song, _L’homme armé_ (‘The Armed Man’), which nearly every Netherland master from Dufay[17] to Palestrina wove with infinite skill into the texture of at least one mass, Josquin des Prés, indeed, into two. If the composer wished to conceal the source of his subject, for the ecclesiastical authorities naturally frowned upon the practice of using secular melodies, or if he invented an original subject, as he occasionally ventured to do, he affixed the title _sine nomine_ to his mass. If it had some uniform peculiarity of construction it was called _Missa ad fugam_ or _Missa ad canones_. Sometimes it would take its name from the number of voices for which it was written, as _Missa quatuor vocum_, or from the mode in which it was composed, as _Missa secundi toni_, or _Missa octavi toni_. Occasionally the subject would be constructed upon the six tones of the hexachord and the work entitled _Missa ut, re, mi, fa, sol, la_; or upon some practice-phrase from the choir-room, as Josquin’s _Missa la, sol, fa, re, mi_. [Illustration: Music score] _L’homme armé_