Skip to Main Content

MIT Libraries logo

MIT logo Search Account

Library of Congress Call Numbers: Overview

What is a call number?

  • It's like an address.
  • It determines the book location (or other item) on the shelf.
  • It represents what an item is about based on a classification system.

The MIT Libraries use the Library of Congress Classification System when assigning call numbers to books and other items in our collections.

For information about music-related call numbers, see more about music call numbers.

Parts of a call number

On an actual item, the number will usually appear on a label like this:



HD --The first part of the call number is alphabetic. To find it, locate the H section, and then the part within it labelled HD. For example, HD would come before HG and after HB.

1234.23 --The second part is numeric and should be read as a whole number. The numbers may or may not have decimal points; but when they do, treat them as such. For example, HD1234 comes before HD1234.23 and HD1233 comes before HD1234.

.P98 --This part has to be read in two ways; first alphabetically, then numerically. This time the number should be read as a decimal!

  • HD1234.23.P would be located between HD1234.23.L and HD1234.23.T, for example.
  • HD1234.23 .P98 would be somewhere between HD1234.23.P961 and HD1234.23.P998.

Some call numbers have yet a third alpha-numeric line. Read the number as a decimal.

1992--This number indicates the year; generally the year of publication for that particular volume.

Call numbers in Search Our Collections

When using Search Our Collections, the MIT Libraries online catalog, a call number usually accompanies the record of an item. In the catalog, the call number appears on one line, like this:


Library Collection Call Number
Dewey Library Stacks HD1234.23.P98 1992


This represents the MIT library in which the title can be found. In this instance, the Dewey Library.
*Please note that there are 5 libraries in the MIT Libraries System, in addition to the Libraries Storage Annex. Be sure to make a note of the library for the title found before you visit the library to look for the item.

This represents the location within the library where the item is located. In this example, the item is in the Dewey stacks. If you are unsure of the location, please ask library staff for assistance.

This provides the specific location of the item on the shelf. The call number is usually found on a label at the bottom of a book's spine or on the front of the book.

More ways to get help


Ask Us
Ask a question, make an appointment, give feedback, or visit us.