Leiden, University Library, BPL MS 2888 (Italy, 13th century), Photo Julie Somers, Turning Over a New Leaf (CC BY-NC-ND 2.0)
A medieval approach to intertextuality, also called a "square bracket" text. The main text was laid in the middle, but with significant margins. The margins would then be filled with the annotations of scholars.
In the photo you can see this executed with a text of the Justinian code, with annotations forming the square brackets. In the middle margin a reader has scibbled some notes of their own.
See also Aristotelian Notes