The article formulates the principles of discrepancies knots formation in preparing material for the classification of manuscripts. The author summarizes the rules of singling out discrepancies knots previously proposed by textual critics and applies them to the text of the Church Slavonic translation of the Gospel of Matthew of the XI-XVI centuries. The applicability of the rules is substantiated by the method of Alekseev cluster analysis. This study allows formulating a definition of discrepancies knot length as a minimum element of the text, the change of which is not followed by the change in other elements.