The Irmos (from the Greek word for "bond/connection") is the first troparion of each ode of a canon, and at the same time is the model followed, both as to meter and melody, by all the following troparia of the corresponding ode of a canon. Each such troparion is supposed to have the same metric pattern, be of an identical length, and have the same musical motifs, as the irmos. The ancient rules for the composition of canons state that the troparia must have (in each verse) an identical number of syllables and the same accents as in the corresponding verse of the irmos. Of course, all of this was observed in the original; but with translation into another language these rules were impossible to maintain. Knowing the chant melody of an irmos, one can correctly chant the troparia of the canon too, since in antiquity, and even today, in the Orthodox East the custom of chanting the entire canon has not been utterly abandoned. In Russia, the chanting of the full canon was retained only for the paschal canon. However, in the Lavra of the Caves of Kiev, before the 1917 Revolution, the canon of Palm Sunday was also chanted in full, as was the penitential canon of St. Andrew of Crete. In the Skete of St John the Baptist attached to the Hermitage of St Nilus of Sora, all the festal canons were chanted. At one time, the canons were chanted at the Hermitage of Sarov; and every week the canons of the cell rule were sung at the Holy Trinity-Hodigitria Convent in the Diocese of Moscow. It is probable that this custom was also observed in many other Russian monasteries; yet we do not now have reliable information concerning this [this article was written in the 1970s]. A vestige of the chanting, as opposed to the reading, of the canon may be found in many of our liturgical books published before the 20th century: in them both the irmoi and the troparia are printed using the same font; but beginning in the 1890s, they began to print the irmoi in a size of print different from the one they used for the troporia.
The irmos is principally the model for the chanting of the troparia, and often in the liturgical books is indicated as a pattern only, and not to be chanted. Thus, in certain editions of the Euchology (Trebnik) of the Lavra of the Caves of Kiev, in the Rite of Unction only the opening words of the irmoi of the canon are provided. This means that one is supposed to chant (or read) only the troparia of the canon, and omit the irmoi. At the end of the Horologion there is a "Supplicatory Canon to the All-Holy Theotokos" (the Little Paraclesis, composed by the Monk Theosterictus). In it, it states plainly: "And the canon of the Mother of God, without irmoi." In the Octoechos (at the end of each volume) there is printed a "Special Canon of Supplication to the All-Holy Theotokos" (the Great Paraclesis), after the conclusion of which one finds printed: "Be it known that on the Holy Mountain, during services of supplication, they do not chant the irmoi of all the odes, but only the irmoi at the beginning of Ode VIII and Ode IX; and at Compline also [they chant] the canon without irmoi, just as at a service of supplication." Why this is so we will endeavor to explain.
The canons are an integral part of Matins, Compline, Resurrectional Nocturns, the majority of services of supplicaton, the Rite of Unction, and, at the present time, the funeral services. At the time the rites of our divine services were being formulated, the place in Matins later occupied by the canon was occupied by the biblical odes; with the passage of time, they began to insert short hymns (troparia), dedicated to festal events, in between the verses of these odes. This is why the "irmos"—the bond/connection—became necessary, to connect the Old Testament content of the biblical odes with the New Testament content of the troparia of the Christian hymnographers (the derivation of the canon is set forth in detail by Professor Skaballanovich in his book The Typicon Interpreted, Part I [Kiev, 1910], pp 368-370; Part II [Kiev, 1913], pp 255-293).
The Irmologion provides three distributions for the nine biblical odes: 1) for weekdays—"Unto the Lord we sing…" (which are the opening words of Ode I); 2) for Sundays, feasts, pre-feasts and post-feasts—then Ode I opens with the words "Let us sing unto the Lord…"; and 3) for the Great Fast, the odes are provided in full (this is also set forth in the Liturgical Psalter).
During the Great Fast, except for Saturdays and Sundays, the biblical odes are performed in full.
For weekdays, they are abbreviated, and for festal days they are further abbreviated. Thus, the indications in the Typicon—"We employ the verses "Unto the Lord we sing…", or "We emply the verses "Let us sing unto the Lord…"—refer to the performance of a festal canon, a weekday canon, or a lenten canon.
In those cases when a canon is chanted with the biblical odes, the irmos is not chanted, but only the troparia. For this reason, neither during the Parastas (the Great Pannikhida), nor during the funeral services, nor at services of supplication, must the irmoi be chanted, but only the katavasiae after those odes when litanies are to be intoned. During the chanting of the canon of the Rite of Unction, the litanies are omitted, and for this reason the katavasiae are also omitted. There are, of course, exceptions, such as the Penitential Canon of St Andrew of Crete, which is performed without the biblical odes but with irmoi (yet it is indicated that the three-ode canons performed after the corresponding odes of the penitential canon are done without irmoi), the Triodia of the pre-feasts of the Nativity of Christ and Holy Theophany, and of Passion Week, are chanted with irmoi, but this is because in these canons the irmoi lose their Old Testament character and, by their content, refer directly to the content of the troparia of the canon.
Thus, as a rule, one should accept (with the above-noted exceptions) that if a canon is not chanted at Matins, its irmoi are not sung.