The antiquity and continuity of the art of illumination has led to many formalist studies in this field. In addition to formalist studies, some traditionalist scholars, such as Nasr, have offered mystical and tasteful interpretations of illumination. He considers illumination as an intermediate art of feminine and masculine arts. Despite a large number of studies in the field of illumination, no study has been done on the relationship between the type of illumination and the content of the manuscript (genre of manuscript). Therefore, the aim of the present study is to select two separate groups in terms of genre and then determine the structural features of illumination in each of the two groups (Khamsahs and Shāhnāmas) based on quantitative characteristics and finally explain the reason for the differences in illumination in these two groups. The features that were selected to examine the illumination are: 1- Examining the rectangular type of illumination frame. 2- Check the width of the illumination frame. 3- Examining the structure of the central part of illumination. 4- Examining the structure of the illumination frame. As a result, in order to achieve this goal, the following questions were answered: What was the evolution of the structure of double-page illuminated frontispiece in the tenth and eleventh centuries AH? What were the main features in the structure of double-page illuminated frontispiece of Shāhnāmas and Khamsahs in the 10th and 11th centuries? And how can their structural differences be explained?
The studies in this article, which have been done by descriptive-analytical method and the necessary information is taken from library sources, show that, in the first half of the tenth century, illumination of double-page illuminated frontispiece in both groups (Khamsah and Shāhnāma) often had a geometric structure, simple frames with a small or normally width, and from the second half of the tenth century non-geometric structures along with geometric structures became common in double-page illuminated frontispiece. Subsequently, the frames have become wider, although these changes are seen more in the Shāhnāmas than in the Khamsahs. And the reason why non-geometric structure is used in illumination of Shāhnāmas much more than illumination of  Khamsahs is due to the difference in the type of text written on these pages. The text in these double-page illuminated frontispiece in Shāhnāmas is often prose (introduction of Shāhnāma) while in Khamsahs it is poetry. It can be said that the type of text written on these pages has influenced the gilded artist in how to choose the type of structure. If in Khamsahs, due to the need for more space for writing the poetic text, the illuminator used more geometric structures and for Shāhnāmas, due to the possibility of writing the prose text in a more limited space, the illuminator also had the opportunity to use non-geometric frames. In the eleventh century, this style of decoration continued in the Shāhnāmas, but their use in the Khamsahs has greatly decreased, and in the Khamsahs produced in the eleventh century, simpler styles have replaced the previous style.