Imposition explained

The rule is simple once you see it: the two pages sharing a sheet always add up to the same number.

Take four sheets of A4, stack them, fold the stack in half. You now hold a sixteen-page A5 booklet. Number the pages as you read them, then unfold: page 1 was sharing a side with page 16. Page 2 was sharing with page 15.

The arithmetic

For a booklet of N pages, the two pages printed side by side always sum to N + 1. On a sixteen-page booklet every pair sums to 17: 16+1, 2+15, 14+3, 4+13, and so on inwards.

Note that the high page is on the left of the front side and the low page on the right. Get that backwards and the booklet reads back to front — which is, incidentally, exactly what happens when someone folds the stack the other way and concludes the printer is broken.

Why not just let the printer do it

Many photocopiers have a “booklet” finishing mode and it often works. Two reasons parishes end up doing it themselves anyway: the copier’s booklet mode usually wants the source document to be A5 pages in reading order, which a Word document assembled by hand rarely is; and the mode is frequently locked behind a finisher unit the parish does not have.

Producing an already-imposed PDF removes the question. The file is A4 landscape, in the right order, and any printer that can do double-sided will produce it.

Margins and the fold

The inner margin — the one against the fold, called the gutter — needs more room than the outer, because the fold consumes a few millimetres and because the eye needs somewhere to rest. On A5, 15mm inner against 12mm outer reads comfortably. Text running into the fold is the single commonest fault in home-made booklets.

Questions

Related