The main feature of the notation used in the Library of Congress Classification (LCC) system is that it combines letters and numbers to represent different subjects and subtopics. The letters generally represent the main class or subclass of the subject, while the numbers represent more specific subtopics within the class or subclass. For example, the notation “QA75.5” might be used to represent the subclass of mathematics dealing with numerical analysis. The use of both letters and numbers in the notation allows for a more detailed and specific organization and classification of materials, which makes it easier for patrons to find what they are looking for in a library that uses the LCC system. The notation is also hierarchical, with more specific subtopics being represented by numbers that follow the letters representing the broader class or subclass.