THIS is not a treatise on Plain Chant, but a compilation of practical definitions and rules which, if rightly understood and faithfully followed,will enable a choir to render well the beautiful melodies of the ancient chant of the Church. But, however well the theory of the chant and the rules of its interpretation may be known, it will not be possible for any choir to obtain really good results, unless there be held regular and fairly frequent practices. And if it be felt that all these rules and practices are too much of a burden and an ever recurring impediment to the freedom of devotion, let it be well borne in mind that the Divine Office is primarily ^.public act of Divine Worships and that consequently private devotion, though by no means a negligible matter, should nevertheless be subordinated to whatsoever the perfect performance of the public act may demand. In point of fact this should not be found too exacting. In this collection of rules and definitions there is nothing, it is hoped, which is contrary to the Regulae Cantus to be found in ourDomimcan Processionarium. (1913 ed.) The author takes this opportunity of expressing his gratitude to Father Clement Donovan, O.P., of the United States Province, for his great kindness in giving much valuable help andfor revising the manuscript.