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Twenty-Percent Rule (LCSH)

LCSH Twenty-Percent Rule

Twenty-percent rule (LCSH 20% rule) is an instruction in the Library of Congress Subject Headings (LCSH) that states to assign headings only for topics that comprise at least 20% of the resource.

  • General Rule
  • General Topic and Subtopic; Principle vs. Specific Case
  • Named Entities
  • Number of Headings
  • Order of Headings 

LCSH headings assignment guidelines and examples are given below in the light of LCSH 20% rule. When subject cataloging follow these instructions to select the most appropriate subject headings for your catalog record.

General Rule

As a general rule in LCSH, the cataloger should assign to the work being cataloged one or more subject headings that best summarize the overall contents of the work and provide access to its most important topics. But LC practice according to this instruction restricts the assignment of headings only for topics that encompass 20% or more of a resource.

20% rule can be very helpful in keeping the subject headings focused on the major topical components of a work. The twenty-percent rule may also apply to supplemental content or notable bibliographic features, such as bibliographies, maps, images, and so on. In the case of a work containing separate parts, for example, a narrative text plus an extensive bibliography or a section of maps, or a book with accompanying materials, such as a computer disc, assign separate headings for the individual parts or materials if they constitute at least 20% of the item and are judged to be significant.

Note: There are certain works to which the Library of Congress assigns no subject headings because of their very general or amorphous nature, for example, a general periodical or a collection of essays with no discernable theme. In addition, it is Library of Congress practice not to assign subject headings to texts of sacred works or to individual works of belles lettres with no identifiable theme or specific form.

General Topic and Subtopic; Principle vs. Specific Case

If a work discusses a general topic with emphasis on a particular subtopic, or presents a principle and illustrates the principle with a specific case or example, assign headings for both the general topic or principle and for the subtopic or specific case or example, provided that the treatment of the latter forms at least 20% of the work.

Example: Title: Revolutions yesterday and today. [A survey of revolutions with emphasis on the Cuban Revolution of 1959]

650Subject Added Entry-Topical Term#0$a Revolutions $x History.
651Subject Added Entry-Geographic Name#0$a Cuba $x History $y Revolution, 1959.

Example: Title: Peoples of the world, in color. [The work is on Ethnology and contains pictures which comprise 20% or more of the resource]

650Subject Added Entry-Topical Term#0$a Ethnology.
650Subject Added Entry-Topical Term#0$a Ethnology $v Pictorial works. 

Named Entities 

Assign headings from either the name authority file or subject authority file for individual persons, families, corporate bodies, projects, events, buildings, named products, uniform titles, etc., that are significant to the contents of the work. Assign headings of this type when these named entities are critical to the subject of the work as a whole, even if discussion of them does not form 20% of the work. 

Number of Headings

The number of headings that are required varies with the work being cataloged. Sometimes one heading is sufficient. Generally, a maximum of six is appropriate. In special situations, more headings may be required. Library of Congress practice, though, is to use no more than 10 headings. In other words, one to six is fairly standard, but a cataloger may go up to 10 headings if necessary.

Order of Headings

If more than one heading is assigned to a work, the headings should be ordered according to their importance. The cataloger shall assign the predominant topic(s) first, followed by secondary topics. 

  • 20% Rule 
  • LCSH 20% Rule 
  • LCSH Twenty-Percent Rule


  1. Library of Congress, "Assigning and Constructing Subject Headings H 180," (accessed December 15, 2018).
  2. Daniel N. Joudrey; Arlene G. Taylor; David P. Miller, Introduction to Cataloging and Classification, 11th Ed. (Santa Barbara, California: Libraries Unlimited, 2015).


Article Title
  • Twenty-Percent Rule (LCSH)
Website Name
  • Librarianship Studies & Information Technology
Last Updated
  • July 03, 2019
Original Published Date
  • December 15, 2018

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Some interesting comments we received on Facebook, Linkedin etc. are given below:
  • Melissa A. Hofmann -- I often violate this to increase access (locally) especially with literature. I usually will increase access for those topics for which I am doing research instruction for a class. 
  • Tabitha MH Green -- I definitely do the same for our fiction collection.
  • Aaron Kuperman -- Cataloger judgment (a.k.a. "cheating" is allowed, and common sense is called for). For example, if I have a book on the law of the USA, UK, Ireland, Canada, Australia and New Zealand, I'm content with "$ English-speaking countries", but if the book included an another country that we don't get many books on (e.g. an African country), I'ld include the less common one. One also has to realize that many authors include multiple topics so if you have three topics and six countries, you end up with too many subject headings (I think 10 is the limit).
  • Alison Bridger -- For Archival collections I violate this all the time. 10% of the resource might actually be a whole box of materials. I have also certainly added more than 10 subject headings.
  • Carol Denehy -- You have to remember that LC rules are written for LC (and kicking and screaming, for PCC). LC does catalogue archive collections, nor is it interested in giving subject access for fiction because of the needs of its patrons. The only establish headings for items they are cataloguing for their own use. You guys need to keep contributing headings and doing things for your public as long as it doesn’t seriously upset the rest of the cataloguing world. I will not get off my soapbox
  • Thom Pease -- And not always true as to music resources (especially classical music), as each item has specific form/genre and medium terms embedded in the LCSH term.