Work letters are a letter added after the date to distinguish various editions or reproductions from the original
The Work Letter "b"
LC uses work letter "b" for editions; that is, same author and title, appearing in the same class, with the same cutter number assigned, published in the same year. This happens often in literature, with, for example, a New York imprint and a London imprint; also, with a large print edition published in the same year as the regular edition.
The Work Letter "a"
LC uses the work letter "a" for :
Photocopies or "true" facsmiles. They use the same call number as the original work, with the date of the original work, plus the work letter "a", so that the photocopy or facsimile will shelve directly next to the original work.
Works entered under a corporate body (110). The first work published in a particular year gets that year added to the call number; subsequent works published in the same year get work letters starting with "a".
Works entered under a conference or congress (111, or sometimes 110). If a date is present in the heading, LC uses that date in the call number; subsequent works with the same heading get the same date with work letter starting with "a"
"Dump numbers". This term refers most often to a cutter used in which no way represents the main entry. An example is the biography table, where A3 represents an autobiography classed in an individual biography number. If two autobiographies by the same person were published in the same year, LC uses the "a' to distinguish them (e.g., .K56 A3 1997 ; .K56 .A3 1997a)
N class: exhibition catalogs or collections of an artist. When two of either occur in the same year, LC's practice is to consult the .xA4 part of the table for special artists on the first cutter; the second work published in the same year would take work letter "a" tacked onto the date (e.g. .V36.A4 1996; .V36.A4 1996a). In these cases, LC doesn't use translation number (e.g. they use.A4, not A413 for English translations of exhibitions catalogs).