The two rules of Christ's birth: [...] In the book of the Church, called the Psalter, there are nine hymns, eight from the Old Testament and the last one from the New Testament. These biblical hymns of the Psalter serve as models for the theme and style, and for this reason, the poets take a word or phrase from the corresponding hymn of the Psalter for each hymn of the Canons.

Note: Since two important Canons have been written for Christ's Birth, one by Cosmas the Maioumas, called the prose Canon, and the other by John of Damascus, called the iambic Canon, the hymns of the Canons of Christ's birth are double, meaning they consist of one hymn from the prose Canon and one from the iambic Canon. As they are sung in pairs, they make the two Canons one with double hymns. For the metric of the iambic Canon, see also the epimetro of the Stichourgiki.

(from the preface of the book)