1. The Dewey Decimal System (DDC): This system categorizes books by subject using a numerical classification scheme. It's widely used in libraries around the world, particularly in the United States and Canada.
2. The Library of Congress Classification (LCC): This system uses a combination of letters and numbers to organize books by subject, with more specificity than the DDC. It's favored by larger research libraries and is commonly used in academic institutions.
These two systems are the dominant methods for cataloguing books globally. While other systems exist, they are less widespread.