Final answer:
The Dewey Decimal System is used in libraries to assign call numbers to books based on subject categories.
Step-by-step explanation:
If you've used a public library or a school library, you've probably found books using the Dewey Decimal System, which assigns specific call numbers based on a number system. The call numbers indicate a subject category. The Dewey Decimal System organizes books into ten main categories, each represented by a specific number.
For example, books on Mathematics are assigned numbers starting with 500, while books on History are assigned numbers starting with 900. This system helps to easily locate books in the library based on their subject matter.