The gutter ensures that text doesn't disappear into the "fold" (the spine) of a bound document.
: In software like Word or Google Docs, you use "mirror margins" so the gutter stays on the inside edge of every page. 🛠️ The "Gutter" in Digital Design & Code GUTTERING
Beyond physical books, the term is used in digital layouts to manage "white space." The gutter ensures that text doesn't disappear into