The Dewey Decimal System is how the library arranges books on the shelves. Each book has a call number with all of the books on similar subjects/topics grouped together by this number.
The call number is the book's "address" in the library so that you can find it easily. The call number has 2 parts: the first part being a 3-digit main number with a decimal that will include additional numbers such as 791.43 on the spine of the book. The second part is called the "cutter"; this is generally the first three letters of the author's last name and will be listed on the spine under the call number.
When looking for books in the library, be sure to document the FULL call number (numbers AND letters) from PRIMO.
Numbers 0-700 are on the main floor, and 800-999 are on the mezzanine. If you are having trouble finding a book by it's call number, feel free to ask for help at the Information Desk.