The Library of Congress call number system is an alpha-numeric system. To read the call number, note the first element or line is a letter or letters of the call number: "A" would be shelved before "AB", "AB" before "AC", etc. Numbers follow the initial letter or letters. The second element or line of numbers are read as "whole numbers" (greater than or equal to 1). A call number of "AP2" will be shelved before "AP4". Therefore, the following items are in call number order:
| AC | AF | AP | AP |
| 5 | 34 | 2 | 4 |
The second element or line is a whole number (greater than or equal to 1) that can be followed by a decimal number (less than 1). Remember, regard whole numbers first than decimal numbers. Therefore, the following items are in call number order:
| DA | DA | DA | DA |
| 345 | 345 | 345 | 345.12 |
| .C35 | .C498 | .C5 | .C12 |
The third element or line of the call number is called the Cutter number. The Cutter is a letter followed by a number that should be read as a "decimal number" (less than 1). A Cutter number "C5" is read as "C.5" and would be shelved after a Cutter number "C498". Remember, regard the Cutter number alphabetically and then by decimal number. In addition, anytime you see something that looks like a zero it is really the letter O. The third line will never start with a number. Therefore, the following items are in call number order:
| DA | DA | DA | DA |
| 345 | 345 | 345 | 345 |
| .C1269 | .C498 | .C5 | .O11 |
Sometimes there are two Cutter number lines. The fourth element or line is a continuation of the third element or line. Remember, regard the Cutter number alphabetically and then by decimal number. Therefore, the following items are in call number order:
| DA | DA | HQ | HQ |
| 345 | 345 | 1213 | 1213 |
| .C498 | .C498 | .W345 | .W345 |
| P127 | P89 | L6789 | L92 |
Dates may often be added at the end of the call number. These dates usually reflect the publication year and can indicate different editions.
