The manuscripts housed at Chartres since the Middle Ages were not all copied and illuminated there. For example, the canons of the cathedral bequeathed many law manuscripts to the chapter, some of which were made in Italy, such as ms. 150, which contains the Decretals of Innocent IV. In order to define the stylistic characteristics of Chartrain illumination, examples must be gathered together by comparing manuscripts still extant at Chartres (or known through old photographs) with those that were made at Chartres but which are now dispersed worldwide. The study of the ornamental decoration plays a primary role in identifying new manuscripts, especially the modest penwork initials, with their ink-drawn motifs, which form a repertory that is typical of twelfth-century Chartrain manuscripts.
The decorative elements ought not to be used in isolation for dating and localizing a manuscript. The script, the nature of the text, the history of its transmission, its recipient and later owners must all be taken into consideration. To illustrate this point, it is useful to remember that in the early twelfth century the Counts of Champagne were at the same time the counts of Chartres and Blois, which explains why, during the 1140s, the same artist who illuminated the Heptateuchon of Thierry de Chartres also painted two Bibles that were undoubtedly ordered by Count Thibaud II for himself and Saint Bernard.