What a mathematics book looks like, 17c. style (a page from the 1670 edition of Diophantus’ Arithmetica; bigger picture here):
So now you’re wondering why I’m bringing this up at all, other than my weakness for ye olde beec (that is, old books). Well, I’m merely drawing your attention to the process of canonization. Wikipedia has another image, of that famous margin which was too small to contain Fermat’s alleged proof of his ‘last theorem’:
A commentative gloss moves from the margin (because all the cool kids are into marginalia) into the very body of the text.
P.S. Long live wide margins.