Edge index

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.

Description
Edge indices are forms 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. When each edge index mark labels one chapter, the desired one can be found by counting the marks. When the edge index marks label first letters in a dictionary or telephone book, some can be identified by their "thickness", (e.g., in English there are only few words beginning with "Q", but many beginning with "S").

Advantages and disadvantages
Edge indexing has several advantages: It is part of the printing process, allows near unlimited headings, and does not add to the binding costs. Its main disadvantage is generally being unable to know what a mark refers to without opening the book, though a coloring scheme may be employed to minimize this downside.

Terminology
An edge index is distinct from a thumb index, but the terms thumb index or chapter thumbs have been applied to edge indices.