Didascalicon, Book IV: the order and number of the divine books
Chapter X. On the canons of the Gospels
The canons of the Gospels Ammonius of Alexandria first devised; afterwards Eusebius of Caesarea followed and composed them more fully. They were made so that by them we may find and know which of the other evangelists have said similar or proper things. They are ten in number, of which the first contains the numbers in which four said the same: Matthew, Mark, Luke, John. The second those in which three: Matthew, Mark, Luke. The third, in which three: Matthew, Luke, John. The fourth, in which three: Matthew, Mark, John. The fifth, in which two: Mark, Luke. The sixth, in which two: Matthew, Mark. The seventh, in which two: Matthew, John. The eighth, in which two: Luke, Mark. The ninth, in which two: Luke, John. The tenth, in which each of them said certain things of his own. The explanation of these is this. For beside each evangelist a number is affixed to the chapters, under which numbers a certain space is marked at the bottom, which indicates in which canon the number is placed to which the space is subjoined. For example: if it is the first space, it is in the first canon; if the second, in the second; if the third, in the third; and so in order you will arrive at the tenth. If, then, having opened any Gospel, you wish to know which of the other evangelists have said similar things, you will take the adjacent number of the chapter and seek that number in its canon, which it indicates, and there you will find who and what they said; and so at length in the body of the text you will find the places sought, which are indicated by the numbers themselves, in each Gospel, where they spoke of the same things.