Book publishing people tend not to talk about leaves very much. We think in pages, and occasionally in spreads. The leaf is a word used by bibliographers, antiquarian booksellers or collectors. A leaf is two pages, front and back, 1 and 2. A 256-page book will thus contain 128 leaves though nobody in a publishing office would have such a thought. Just about the only situations I can think of in which we publishers might refer to a leaf would be when referring to something overleaf, i.e. on the back of the page you are looking at, when talking about leafing through a book, or maybe when applying gold leaf to the edges of the book or the case stamping. You might occasionally hear the word interleaved: a book, often a bible, which has has blank pages (a blank leaf) tipped in between the pages. An extreme form of interleaving is illustrated by Grangerizing.

Leaves form the basis for the old-fashioned description of book sizes: quarto, octavo etc.

A spread can be an important consideration in the design of a book. By spreads, a publisher would be thinking of two facing pages, a verso plus a recto, 2 plus 3. You may want to place a picture with one half on one page and the other half on the facing page, so it can be shown as large as possible. So designers will be working with the idea of a spread being an even-numbered page followed by the next odd-numbered page. For the printer though the spread will look very different — in other words the left hand bit of the picture will not be next to the right hand part when the job gets on press. This is a consequence of imposition.

One of the hazards of designing across an opening, a spread, is that the alignment in the final book becomes a hostage to the accuracy of the fold. You can see it happening in this photo of a spread from Passage by Andy Goldsworthy, printed most excellently by Artegrafica in Verona. Folding machines are pretty much guaranteed not to achieve perfect alignment, so it’s generally wise not to make the picture you spread across an opening be one which contains strong horizontals: The Andy Goldsworthy picture doesn’t represent a big problem, which of course is why they decided to do it that way, but you can see the slight misalignment at the bottom of the picture in the gutter fold. However if you run a rule all the way across pages 2 and 3 you are bound to find a broken rule, one half up to 1/16″ above the other. Incautious designers have all too often fallen into this trap by placing a rules below the running head and folio and running them into the gutter in the hope that they’ll align with the facing page’s rules. They won’t. (Except of course on the two pages at the center of the section.)

Bethart Printing provides a helpful illustration of the difference for an 8-page booklet.

A larger job would obviously need to be scaled up, but this little example makes the point. In their heading at the top of the right hand grey area they mean Printer’s Spreads of course. They elide the fact that pages 8 + 1 and 6 + 3 print on one side of the sheet with 2 + 7 and 4 + 5 backing them up. 4 + 5, as you can see, form the center of the section, and anything printed across both pages will align perfectly.