An _ode_ is a song or hymn compounded of several similar "troparia,"--usually three, four or five. To these is always prefixed a typical or standard "troparion," called the _hirmus_, by which the syllabic measure, the periodic series of accents, and in fact the whole structure and rhythm of the stanzas which follow it are regulated. Each succeeding "troparion" in the same "ode" contains the same number of verses, and of syllables in each verse, and similar accents on the same or equivalent syllables. The "hirmus" may either form the first stanza of the "ode" itself, or (as is more frequently the case) may be taken from some other piece; and, when so taken, it is often indicated by initial words only, without being printed at length. It is generally printed within commas, after the proper rubric of the "ode." A hymn in irregular "stichera" or stanzas, without a "hirmus," is called "idiomelon." A system of three or four odes is "triodion" or "tetraodion." A _canon_ is a system of eight (theoretically nine) connected odes, the second being always suppressed. Various pauses, relieved by the interposition of other short chants or readings, occur during the singing of a whole "canon." The final "troparion" in each ode of the series is not unfrequently detached in sense (like the "ephymnia" of Ephraem Syrus), particularly when it is in the (very common) form of a "theotokion," or ascription of praise to the mother of our Lord, and when it is a recurring refrain or burden. There were two principal periods of Greek hymnography constructed on these principles--the first that of Romanos and his followers, extending over the 6th and 7th centuries, the second that of the schools which arose during the Iconoclastic controversy in the 8th century, and which continued for some centuries afterwards, until the art itself died out. School of Romanos.