IRMO (εἰρμός). – It is a rhythmic composition of a chant according to a given metre, which serves as a model for joining (εἴρω) to it a series of strophes or troparia in the same rhythm. The collection of these model melodies gave rise to the liturgical book called the Heirmologion (Εἰρμολόγιον). The i. is of the greatest importance in the composition of the long hymns called “canons” (κάνονες), which contain eight or nine odes (ῥᾶσι). Each of the strophes is sung according to the modulation expressed by the i. placed at its head. Even for isolated troparia, the i. serving as its model is indicated in a few words.
BIBL.: I. B. Pitra, Hymnographie de l'Eglise grecque, Rome 1867, p. 29 ff.; W. Christ – M. Paranikas, Anthologia Graeca carminum christianorum, Leipzig 1871, p. 60 ff.; E. Bouvy, Poètes et mélodies, Nîmes 1886. Placido de Meester