Imposition: why page 1 sits beside page 8
Fold a sheet three times and the page order stops being obvious. Printers solved this before 1500.
A book is not printed a page at a time. It is printed on large sheets, each carrying many pages, which are then folded, cut and gathered. Working out which page goes where on the sheet, and which way up, is called imposition, and getting it wrong wastes the whole run.
Take one sheet folded three times: eight pages a side, sixteen in all. On the outer face, page 1 sits beside page 8, because after the fold those two ends meet. Half the pages are printed upside down relative to the other half. Hold a cheap paperback up to the light at the spine and you can still see the logic.
Why it survives digital
Because folding survives. A book printed today on a digital press is still imposed, still folded into gatherings, still trimmed on three edges. The arithmetic did not change when the plates did.
The part that bites
Creep. The pages at the centre of a folded gathering stick out further than the ones on the outside, because they travel around more paper. On a thick gathering the trim removes more margin from the inner pages than the outer, so the margins you drew are not the margins you get.
The fix is to shift each page slightly inward as it approaches the centre of the gathering. Every imposition tool does this and every first-time self-publisher discovers it after the proof arrives.