Here at Penn, as with most academic libraries, we use the Library of Congress classification system to arrange our materials. LC uses a combination of letters and numbers to create a Call Number unique to each item. 

You can think of a call number as a book's address on the shelf. The first line in a Library of Congress call number classifies the item by its subject according to the LC Classification System. The entire call number should be noted in order to locate the item.

• LC call numbers are first arranged alphabetically, according to the letter or letters at the beginning of the call number, which correspond to the subject matter of the item. Example: Call numbers beginning with P are followed by PA, PB, PC, etc.
• Within the alphabetical section, books are arranged by the number(s) that follow. Notice that these numbers are regarded as whole numbers. For instance, PS 1200 comes after PS 345. Example: QA 56 before QA 234 before QA 234.57 before QA 234.8
• The next lines are alphabetic, then numeric as a decimal. The numerical part of this section is a decimal number, not a whole number. Use alphabetical order first, then the decimal extension to put the call numbers in correct sequence. (A3113 would come before A4, because 0.3113 is smaller than 0.4.) Example: HV 1431 .B7 before HV 1431 .B83 before HV 1431 .F25 before HV 1431 .F7
• Editions are often arranged by date or by the date and letters. Example: PS 3525 .I52 1971 before PS 3525 .I52 1973 before PS 3525 .I52 1973a

Original Source: University of Buffalo Library