Finding material on your research topic will not be easy. Let's say the books are organized by color. If you are searching for books on the Holocaust, you will have to run all over the library, upstairs and downstairs, through all the stacks of books, because the books on the Holocaust may come in many colors. The blue books may be on the first floor while the yellow books are on the second floor. This is not very convenient or efficient.
Libraries use classification systems based on subject to organize materials.
The two most popular classification systems are the Library of Congress and the Dewey Decimal. Call numbers are made up of a combination of letters and numbers. For example, 940.53 G45, is the call number for a book in the UCCC collection. You can consider the call number as the address of the book in the library, and know exactly where to locate it. Since these classification systems are based on subject matter, similar materials are shelved next to each other.