What is Page fill, and what is a river of white?
Open a well-made book at any spread and the text blocks end at the same depth on both pages. That evenness is one of the quiet things that separates a printed book from a printout — and it is not free.
Why it costs something
A page ends level only if the text can be persuaded to fill it exactly. The formatter does that by re-breaking paragraphs and letting some lines carry slightly wider word spaces. Stretch a little and nobody notices. Stretch a lot and two things go wrong: the page starts to look gray and uneven, and the wide gaps on stacked lines can line up vertically into a pale channel running down the page — a river of white.
So it's a trade, and the Page fill setting is where you make it.
The three settings
- Even word spacing. Word spaces stay near their natural width, so rivers are least likely. Some pages end a line or two short of the bottom.
- Balanced (recommended, and the default). Fills about nine pages in ten. A few lines carry wider word spaces, which is what can open a river.
- Flush page bottoms. Every page ends level. Word spaces stretch much further to get there, and that's what opens rivers.
Judge it on your own book, not on the description
The three settings behave very differently from one manuscript to the next — it depends on your sentence lengths, your vocabulary, and how wide the measure is at your trim. That's why the formatter counts the result on your book rather than asking you to trust an adjective: switch between the settings and you can see, for your actual manuscript, how many pages end level and how many carry a river.
A book with plenty of short paragraphs and dialogue can often run Flush with almost no cost. A dense book of long descriptive paragraphs may not.
Reading the numbers
The export report gives you both figures for the setting you exported at:
- Pages ending level — the proportion that reach the nominal bottom.
- Rivers — how many pages carry one, and how many lines the longest one runs down.
A handful of rivers in a 300-page novel is normal and nobody will notice. If the count looks high, step down one setting and look again. See What is in the export report?
Pages that end short
Some pages end short no matter what you choose, and that's correct. A chapter's last page ends where the chapter ends. A page that would otherwise strand a single line at the top of the next page gives that line up deliberately — see How are widows and orphans handled?
The report separates these. A page the engine flags as running short of the bottom is one where a few words added or cut in that chapter would close the gap — and the formatter will tell you which chapters those are, so you can fix them as a writer rather than spending word spacing on it.