The Dewey Decimal Classification System uses a number system to classify and organize all books in a library. The system starts with 10 main categories; each include 10 sub-categories, which in turn include 10 topics.
The Dewey Decimal system assigns a three-digit number to each of its 10 main categories. For instance, 000 is for computer science, 200 for religion and 500 for science. Each main category contains 10 sub-categories which start with the same digit. For example, while all sciences are 500, 520 is astronomy, 530 is physics and 590 is zoology.
The sub-categories contain topics which follow the numerical system. All zoological sciences topics begin with 59; 592 is for invertebrates, 593 is for protozoa and 595 is for other invertebrates, including insects and worms.
The topics include specific subject areas. A decimal point comes after the first three digits to separate the specifics. For example, 595.7 denotes insects, 595.78 denotes the order Lepidoptera and 595.789 denotes butterflies.
Libraries assign cutter numbers to books that have such similar topics that they end up with the same classification number. The cutter number is usually the first letter of the author's last name followed by a series of numbers from a library table. The cutter number is on a separate line from the classification number.