IRMOS (εἱρμός). - It is a rhythmic composition of a chant according to a determined meter, which serves as a model to link (εἱρμός) to it a series of strophes or troparia of the same rhythm. The collection of these type-melodies gave rise to the liturgical book called the Irmologion (Εἱρμολόγιον). The irmos has the greatest importance in the composition of the long hymns called "canons" (κανόνες), which contain eight or nine odes (ὠδαί). Each of the strophes is sung according to the modulation expressed by the irmos inserted at its head. Even for isolated troparia, it is customary to indicate with a few words the irmos that serves as the model.
BIBL.: I. B. Pitra, Hymnographie de l'Église grecque, Rome 1867, p. 29 ff.; W. Christ, M. Paranikas, Anthologia Graeca carminum christianorum, Leipzig 1871, p. 60 ff.; E. Bouvy, Poètes et mélodes, Nîmes 1886. Placido de Meester