The 6 Principles of Design
The pages are even, for example theirs not a lot of writing on only one side, its even on both sides.
The distance between elements, its used to show the relationships of one between things
The justification of elements. And emphasizes ones relationship with each other.
Consistent pattern of font and color schemes and graphic types.
the use of color and size to emphasize the most important elements on a page.
the blank or negative space on a page.