Recto is the "right" or "front" side and verso is the "left" or "back" side when text is written or printed on a leaf of paper (folium) in a bound item such as a codex, book, broadsheet, or pamphlet.
In double-sided printing, each leaf has two pages – front and back. In modern books, the physical sheets of paper are stacked and folded in half, producing two leaves and four pages for each sheet. For example, the outer sheet in a 16-page book will have one leaf with pages 1 (recto) and 2 (verso), and another leaf with pages 15 (recto) and 16 (verso). Pages 1 and 16, for example, are printed on the same side of the physical sheet of paper, combining recto and verso sides of different leaves. The number of pages in a book using this binding technique must thus be a multiple of four, and the number of leaves must be a multiple of two, but unused pages are typically left unnumbered and uncounted. A sheet folded in this manner is known as a folio, a word also used for a book or pamphlet made with this technique.
