CANON INNODICUS. — This term designates the rule to be followed in liturgical poetry. In this context, rhythm presents a fixed form or pattern to which the composer must conform. A similar rule or canon was already observed in classical Greek poetry, both in the number of feet and of strophes. However, the tendency toward artistic freedom gave rise to many exceptions or substitutions known as “licentiae poeticae” (poetic licenses). At a certain period, poetic compositions that deviated least from this canon were called “irmos” (v. IRMOS) (ἱρμός).
Liturgical poetry has always had its canons, though some differ from those of classical poetry. In the East, beginning with St. Ephrem, and in the West, especially through the work of St. Hilary and St. Ambrose, poetry and hymnody were employed for popular instruction. St. Ambrose used them to instruct the people in Catholic doctrine and to combat heretical errors; to better suit popular style, he replaced the classical quantitative feet with the accentual stress of words.
Various types of composition have been adapted over time for liturgical hymnody, but the classical model is the eight-syllable line, arranged in strophes of four verses, resembling the “iambic-metrical” form. Of all the canons of innodic poetry, only the number of syllables remains in actual use. It is uncertain whether the melody was originally always strictly syllabic; however, it is certain that very ancient hymns already had more or less ornate melodies, so that the harmonious cadence of eight beats is less perceptible. In any case, at least in recitation, “elision” is observed when there are more than eight syllables, and this practice is also preferred in singing.