An edge index is a form of index that consists of marks on the edges of the pages of a printed work. These marks are step-like printed and usually contain order words, letters, or numbers, (e.g., A to Z in a dictionary or telephone book). Usually, they are colored and help to find desired points, especially in reference works. They are created by printing to the edge of the sheet so that they are visible on the closed books' edge.
Advantageously, edge indexing is part of the printing process, permits nearly unlimited headings, and does not add the cost of binding. These are offset by the disadvantage of being unable to know what a mark refers to without opening the book.