Along three axes:

Form

1. Little number in the corner
2. Little lower case roman numerals in the corner
3. That little cross looking bullshit

Location

1. Proper footnotes, at the bottom of the page
2. Endnotes at the back of the book, at least I know more or less where to flip
3. Endnotes at the end of the chapter, bleh

Content

1. Supplemental or tangential text and other commentary¹
2. Bibliographic references, booooring
3. A mix of the above
4. A mix of the above in combination with being at the end of the book or chapter so I can't even tell if it's going to be worthwhile to flip back and read it, gtfo

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1. Bonus points if you go all Infinite Jest-style sicko-mode with nested and cross-referenced footnotes, oh yes give it to me. ↩