A book has four margins, and they are not equal. The inside margin, called the gutter, must be wider than the others because part of it disappears into the binding, and it must grow as the book gets thicker. A 500-page book needs roughly twice the gutter of a 100-page one.
Everything else about margins is a design choice. The gutter is the one with hard numbers attached, and they come from printers rather than from style authorities.
The Four Margins
- Gutter, or inside margin. The edge nearest the spine. Widest, because the binding eats into it.
- Outer margin. The thumb edge. Where the reader's hand sits and where the folio usually falls.
- Head. The top.
- Foot. The bottom, traditionally the deepest of the three non-gutter margins.
Because the gutter is on the left of a recto and the right of a verso, margins mirror across the spread rather than staying fixed. This is why book layout software asks for inside and outside rather than left and right.
Wiley defines the space they enclose as the type area, "the blank space surrounding the type area" being the margins themselves, and notes the type area "does not include the drop folio". A page number dropped to the foot sits in the margin, not in the text block.
How wide that text block should be is a typographic question rather than a manufacturing one, and it depends on the typeface and size you choose.
Why the Gutter Grows
Open a thick paperback and the pages curve into the spine. The thicker the book, the more severe the curve and the more of the inner margin is lost into the binding. A gutter that works at 100 pages leaves text disappearing into the fold at 500.
Printers publish this as a table keyed to page count. Amazon KDP's, which Deckle follows:
| Page count | Minimum gutter |
|---|---|
| up to 149 | 0.375 in |
| 150 to 299 | 0.5 in |
| 300 to 499 | 0.625 in |
| 500 and above | 0.75 in |
Attribute this rule to printers, not to typography. No professional editorial body and no trade publisher I could find states it. CIEP's editorial glossary does not contain the word gutter at all, and publishes no measurements of any kind. Wiley defines margins qualitatively and gives no figures. The page-count table exists only in printer and platform documentation, because it describes a manufacturing constraint rather than a design principle.
The outer margins carry no equivalent rule. They are a design decision, bounded at the low end by the printer's safety margin: Wiley's print specification asks that "all important type or images should be kept at least 6 mm (1/4 in) inside the trim".
Doing This in Deckle
Deckle exposes four margin settings: top, bottom, inside and outside.
One behaviour you should know about, because it is silent. Deckle enforces the printer minimum for the gutter. At export it takes whichever is larger, your inside margin or the minimum for your page count, and uses that. So setting a 0.4 inch inside margin on a 400-page book gives you 0.625 inches in the exported PDF, with no warning and no visible change to your setting.
That is the safer behaviour, since it cannot produce an interior with text lost in the binding. But it does mean your inside margin setting is a floor rather than an instruction once the book is thick enough, and the number in the settings panel is not necessarily the number in the PDF. If your text block looks narrower than you expected on a long book, this is why.
Deckle also adds bleed to the canvas at export, extending the top, bottom and outer edges while leaving the spine side alone.
Common Mistakes
- Setting all four margins equal. The gutter needs more, and the amount depends on page count.
- Using left and right instead of inside and outside. Margins mirror across the spread.
- Choosing the gutter before the book is finished. It is keyed to final page count, so a late chapter can push you into the next band.
- Treating the printer table as typographic guidance. It is a binding constraint. No style authority publishes it.
- Running content to the edge of the margin. Keep a quarter inch of safety inside the trim.
- Assuming the setting is what gets exported. Deckle raises the gutter to the printer minimum silently.
FAQ
Q: What is the difference between the gutter and the inside margin? A: In practice they are used interchangeably for the margin nearest the spine. Strictly, the gutter is the combined space where the two facing inner margins meet.
Q: How wide should my margins be? A: The gutter is set by the table above. The others are a design choice, kept at least a quarter inch inside the trim.
Q: Does the gutter change if I add a chapter? A: It can. It is keyed to page count, so crossing 150, 300 or 500 pages moves you into the next band.
Q: Do ebooks have margins? A: Not fixed ones. A reflowable file lets the reading system and the reader decide.
Q: Why is my exported text block narrower than my settings? A: Because the page count pushed the gutter above the value you set, and the printer minimum won.