trope

An interpolation into liturgical chants dating from the 8th or 9th centuries, probably of Byzantine origin. Tropes were at first vocalized as purely musical ornamentations or sung on syllables of certain words, especially ‘Alleluia’, but later they became so important that special words were newly written for them. Sometimes new words came first and demanded new music; thus the sequence, which began merely as a special kind of trope, developed into a poetical form with musical setting.