Adjustable bookend stop

A book stopper usually works according to two basic principles. The first principle lies in its weight. It is so heavy enough that it does not move from the place even under the influence of relatively heavy books. The second principle uses the weight of the book itself, when the stopper slips under the book and thus becomes fixed. I decided to use the shelves between which the stopper stretches and locks. It creates a sort of additional column in the library that serves as support for books.