Irmologion (Greek: τὸ εἱρμολόγιον heirmologion) is a liturgical book of the Eastern Orthodox Church and those Eastern Catholic Churches which follow the Byzantine Rite. It contains irmoi (οἱ εἱρμοί) organised in sequences of odes (αἱ ᾠδαὶ, sg. ἡ ᾠδή) and such a sequence was called canon (ὁ κανών 'law'). These canons of nine, eight, four or three odes are supposed to be chanted during the morning service (Orthros). The book Irmologion derives from heirmos (ὁ εἱρμός) which means 'link'. The irmos is a melodic model which preceded the composition of the odes. According to the etymology, the book 'collects' (λογεύω logeuō) the irmoi.