How did DC, Marvel and other comic book publishers keep track of their...

...characters before the Internet? DC, Marvel and other publishers have 100s to 1000s of characters that they create and acquire from thero companies. How did they keep track of all of their characters before the advent of the Internet? I am curious- they must keep records or all of their characters somewhere. I know DC had their Who's Who guides and Marvel had their Handbooks, both of which are targeted to fans. The Internet makes keeping track of things a lot easier, but how did these companies keep tabs on all of their properties before the Internet?

I want to create my own comics- I'd like to know this also.