By writing down who borrowed what, I guess, but does anybody know details? Like was there a big file cabinet with a file for each book, and they had to write a name or ID # of the borrower in the file? Or did they have a big book with a line-entry for each book, with the name/ID of the borrower written next to the book ID? (This doesn’t sound too feasible what with new books always coming in).
I’m guessing it was some system different from the ones I described above, neither of which sound very efficient even for pre-computer days.