If you ever wondered if those indentions in old-school dictionaries have a particular name, the answer is that yes, they do. They are called an index notch.

If you ever wondered if a close-up photo of an index notch* looked like a piece of wood that had been carved out, that answer is also yes.

Lubbock, Texas
photographed 10.27.2028

*See how smart you are? You JUST learned a new word and there you are, already using it in a sentence. Yay, you!