What is a call number?

A call number is just a code given to a book that relates to where it is shelved. The call number will normally look something like this:

QA 173.85 BAH

"QA" refers to the main subject area of the book (in this case mathematics), whilst the number gives a more specific identification of the subject - even further detail is given by the decimal number. Finally, the three letters usually refer to the author's name.

Sometimes the call number is prefixed by "quarto", "pamphlet", or something similar. These refer to specific subsections of stock, and are shelved separately from the main sequence of books.

For instance, "quarto" refers to oversized books that are too large to be conveniently shelved with the main section of books.