The term "page" is confusing since it can mean two different things when referring to double-sided printing: a page or a sheet. We count pages as the page numbers normally run. Each sheet in the book has two pages, one on each side; in fact, each sheet has two pages even if one of them is blank. There are twice as many pages as there are sheets. If the pages in a book are numbered from 1 to 250, then there are 250 pages in the book and 125 sheets. It's really confusing for us when you ask for a quote with a page description of "320 pages printed on both sides". To us that means 320 pages (160 sheets) and they are not blank on the back. If you mean 640 pages when you say this, we won't get your quote right for you