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A series of rounded indentations cut into the front edge of a book, each labeled, as with a letter, to indicate a section of the book.
(Printing, Lithography & Bookbinding) a series of indentations cut into the fore edge of a book to facilitate quick reference
(Printing, Lithography & Bookbinding) (tr) to furnish with a thumb index
a series of labeled notches cut along the fore edge of a book, to indicate the divisions or sections.