[Xorakios]

In school and public libraries it was a physical piece of thick paper, similar to a Rolodex card, which had multiple copies, always one per author and one per title, but sometimes backups by subject manager. And when I was a kid, there were often multiple, redundant copies that one could take to the librarian to check out the physical book if it were particularly important.

Yep, that old :(

[dhosek]

Wait, you had card catalogs where you took the cards out of the drawer? I grew up with card catalogs where there was a metal rod at the bottom of each drawer that ran through holes in the bottom of the cards. There were cards for author, title and subject with multiple subject card possible for each book. The CIP blocks on the copyright pages of books list all the possible subject headings.

And I've been thinking that I don't use the subject indexes on the online catalog anywhere near enough. They can surface things that the standard keyword search might miss or bury among irrelevant results.