Did you know that Markings found on cover pages in books date back to libraries before computers? Back then, library staff created cataloging (to help our users find our books) which had to be typed on cards and filed manually in the card catalog. This means there were limited “retrieval points.” But if we needed to remove a book from the collection, we needed to find and remove all the cards in the card catalog for that book. Every book had at least a main entry card, which was the most important card of all, because it told us how many other cards were in the catalog for that book, and what they were. The three dashes you see on the title pages of books indicate the start of the main entry that was used on that all important card (which is usually the main author or sometimes the title). Frequently, the main entry was obvious, but cataloging rules sometimes made it a bit obscure. The underlining is the start of the title.
We no longer need to follow this practice because technology means we have multiple ways to find and remove the cataloging records from our catalog.