thumb index

7.2

Small cuts in a book edge that help you open to chapters

noun
/θʌm ˈɪndɛks/

Specialized

: índice de pulgar

The reference book has a thumb index so you can quickly find each chapter.

Examples

She prefers dictionaries with a thumb index for easier navigation.
She used the thumb index to locate the recipe.
Adding a thumb index made the manual much more user-friendly.
The thumb index helps readers find chapters quickly.
She prefers dictionaries with a thumb index for easier navigation.
Adding a thumb index made the manual much more user-friendly.
The reference book has a thumb index so you can quickly find each chapter.

Surface Forms

thumb index singular

thumb indices plural

Morphology

thumb + index

The parts 'thumb' (a finger used for pointing/gripping) and 'index' (a locator/list) together suggest a navigational aid you use with your thumb, so learners can infer a relation to finding sections. However, the specific meaning — rounded notches cut into a book’s edge — is a specialized printing/typographic feature not fully predictable from the constituents, making the expression only partially transparent to B1 learners.

Etymology

Thumb index comes from tiny notches cut into the edge of a book where you can put your thumb to open the book right at a chapter. That's why it now means small tabs that give 'quick access' to sections.