The front and back of a single sheet of paper can be referred to as the ‘recto’ (front) and ‘verso’ (back).
"Recto is the term for the 'right' or 'front' side of a printed page, while verso is used for the 'left' or 'back' side. In double-sided printing, each leaf contains two pages – front and back. In modern book production, paper sheets are folded in half, creating two leaves and four pages per sheet.
For instance, in a 16-page book, the first leaf holds pages 1 (recto) and 2 (verso), and the second leaf contains pages 15 (recto) and 16 (verso). Pages 1 and 16, therefore, share the same side of a physical sheet, combining recto and verso sides of different leaves.
To use this binding method, the total number of pages in a book must be divisible by four, and the number of leaves must be divisible by two. Unused pages are often left unnumbered and uncounted. This folded sheet is known as a folio, a term also applied to books or pamphlets created using this technique.