Searching with Library Subject Headings
Library subject headings are different from keywords. You could pick several keywords to describe a topic, but only one of those keywords might be a subject heading.
Subject headings are a controlled vocabulary. They are created by library "experts" at the Library of Congress who look at a topic and choose one standardized word or phrase to describe it. Librarians all across the country use a book, which is basically a list of those standardized words to assign a book its subject headings.
Subject headings are assigned to a book or other media to signify the book's most significant content. For example, a book that discusses what it's like to grow up a twin would be assigned the subject heading, Twins by a librarian when it is cataloged. If you were searching online for information about twins, you could use the keyword, twin, or multiple births and you would get results, but if you were doing a subject heading search in an electronic library catalog, a search using multiple births would not give you any results, since it is not the "official" subject heading for that subject.
So, why would it be advantageous to do a subject heading search when you can just as easily do a keyword search?
Well, let's say you were searching in an online journal database. You are looking for journal articles that really cover your topic thoroughly. When you try a keyword search using the keyword search feature of the database, you get thousands of articles that mention your topic here and there, but nothing with great depth. Of course, you don't take the time to go through the ENTIRE list of results, because there are just too many. If you did go through it, you would eventually come upon those articles with more information.
Searching the same topic and using the subject heading search feature of a database, you would quickly get those in-depth articles on your results list. Why? Because they were assigned the specific subject heading you searched because the greatest part of the article's content is about that subject.
If you are unsure of which of your keywords might be the assigned subject heading, many databases provide a list of the subject headings they assign to the articles.
Subject headings are used in library catalogs, both print and electronic, and in research databases.
Some subject headings are easy, for example, DOGS is the subject heading for dogs; SUNFLOWERS is the subject heading for sunflowers. Some are a bit more complicated, such as AGED for "senior citizens" and HEALERS IN MASS MEDIA for "television faith healers". Why are these subject headings used? It's simple - because the list created by the Library of Congress "list" says so. That's the point with controlled vocabularies. They are created by people out there who "control" them and those people don't let you, the searcher, change them.
Having to use someone else's subject headings gives you less flexibility than coming up with your own keywords, but you are more likely to find what the database has to offer on your topic.
How does a controlled vocabulary work?
Catalogers (librarians who add all of the information about a book, article, etc. to the library catalog) use the selected list of subject headings provided to them by the Library of Congress. These catalogers decide which of the subject headings best describes the content of the book - the primary and secondary subjects as well. This means that a book will have more than one subject heading, if there is more than one subject covered in the book. For example, a book about the media might have MASS MEDIA as a subject heading, but if it covers freedom of the press in book, it may have the subject heading, FREEDOM OF THE PRESS, or FREEDOM, or CIVIL RIGHTS as well.
For example, a book might be titled, Senior Citizen Lifestyles in the 21st Century, and would then be assigned the subject heading, AGED, by the cataloger, as one of its subject headings. So, if you were to do a subject heading search for this book and others like it, you would have to search for the subject heading, AGED in order to find it.
It's important to note that fiction books have subject headings, too!
So, if you are doing a subject heading search on DRUG ABUSE, you need to pay attention to details about the book or its call number, because if the fictious story is about a teenager's struggle with drug abuse, like in Crank by Ellen Hopkins, the book will have DRUG ABUSE as a subject heading. If the book is fiction, you won't be needing it for your research.