050 - LC Call Number

The Library of Congress Classification (LCC) is a classification system that was first developed in the late nineteenth and early twentieth centuries to organize and arrange the book collections of the Library of Congress. It has since been adapted for local use in many other libraries, particularly academic libraries. The system divides all knowledge into twenty-one basic classes, each identified by a single letter of the alphabet. Most of these alphabetical classes are further divided into more specific subclasses, identified by two-letter, or occasionally three-letter, combinations. 

LC call numbers are found in the 050 tag of the MARC record. Subfield a contains the classification part of the call number (indicating what the thing is about) and subfield b contains the item portion of the call number (the who and when of the creation of the thing).

For example, the LC call number for Franklin Pierce by Peter A. Wallner is E432 .W35 2004 which is found in the record as 

050 0 0 ǂa E432 ǂb .W35 2004

The 'who' portion of the call number is a cutter (W35 in the example above). There is an excellent description of how a cutter is assigned in the Harvard University library's cataloging documentation. 

The first indicator tells you if the Library of Congress has the item in their collection and the second indicator tells you who created the classification number. In the above example both indicators are 0 (zero) telling us that LC has the title in their collection (1 would mean the item is not in LC) and that the classification number was assigned by LC (4 would mean it was assigned by an agency other than LC following LC's rules). A blank in either indicator means no information has been provided.