Every library has its own system. But pretty much every system uses a "Call Number" to tell you where in the library that book is found.
Here at HBS, we use a system created for our library.
Step 1: is it...
Fiction Non-Fiction
* tells a story * gives information
* organized by genre (flavor) * organized by subject
* used letters for sections * uses numbers for sections
That will tell you what half of the library the book is in, sort of like the zip code or neighborhood.
Step 2: what sub-section? Sort of like the street name
Step 3: The 3 letter code (usually the first 3 letters of the author's last name) - sort of like the house number