Books shelved using the LoC system are first divided into large sections designated by letters of the alphabet: a section of A's, a section of B's, C’s, D’s, etc., continuing through the alphabet.
Each of these sections is further broken down by adding a second letter to the main section letter; for instance, CA, CB, CC and so forth; a third letter is added in a few cases. Added after the main letter designations, alternating letters and numbers break down subject areas into greater and greater detail.
A simple breakdown of the LoC system is available online* and posted on the end of the shelving in the Reference Collection, showing main letter combinations and the subjects they represent.
* Source: “LC Classification Outline, 7th Edition: Contents.” n.d. Cataloger’s Reference Shelf. The Library Corporation. [“based on LC Classification Outline, Seventh Edition, 2003, Cataloging Policy and Support Office, Library of Congress; Cataloging Distribution Service, US Library of Congress, Washington, D.C.”] Web. 6 Oct. 2015.