Catholic Encyclopedia (1913)/Accentus Ecclesiasticus
The counterpart of concentus. In the ancient Church music all that portion of the liturgical song which was performed by the entire choir, or by sections of it, say two or three singers, was called concentus. Thus hymns, psalms, and alleluias were, generally speaking, included under the term concentus. On the other hand. such parts of the liturgy as the priest, or the deacon, or subdeacon, or the acolyte sang alone were called accentus; such were the Collects, the Epistle and Gospel, the Preface, in short anything which was recited chiefly on one tone, rather than sung, by the priest or one of his assistants. The accentus should never be accompanied by harmonies, whether of voices or of instruments, although the concentus may receive an accompaniment. The words Gloria in excelsis Deo and Credo in Unum Deum, being assigned to the celebrant, should not be repeated by the choir or accompanied by the organ or other musical instrument.