A canon (Greek: κανών, romanized: kanōn) is a structured hymn used in a number of Eastern Orthodox services.
[1] Via his translation of the Jerusalem Georgian Chantbook, Stephen Shoemaker demonstrates that this form of hymn was already in place by the early 5th century, challenging the prevailing model of Byzantine musical development.
At some point, this is interrupted by an introductory stanza called an irmos ("link"), which poetically connects the theme of the biblical canticle to the subject of the canon.
(Katabasia means "coming down" and the verse is so called because as originally performed the two choirs would descend from their places on the left and right sides of the church to sing it together in the middle.)
Theoretically, each ode has fourteen (or occasionally sixteen), with some troparia repeated if the service books do not provide enough of them and some conjoined if there are too many.
Canons are used most notably at Matins, but also at the Midnight Office for Sunday; at Great and Small Compline; and at special services such as the Paraklesis and those of similar structure such as the Panichida and Moleben.
One traditional prayerful preparation for reception of the Eucharist is to read three canons and an akathist the evening prior.
Because the use of triodes is so prevalent during Great Lent, the book containing the changeable portions of services that liturgical season is called the Triodion.
In the Russian Orthodox Church, for arcane historical reasons, the Pentecostarion is called the Flowery Triodion even though it contains no triodes.
The irmoi and katabasia for various occasions are found gathered together in the Irmologion, one of the standard service books of the Orthodox Church.
Complete canons (irmoi with their troparia) are found in the Menaion, Octoechos and Horologion used throughout the year, and in the seasonal service books the Triodion and the Pentecostarion.