A card catalog is a physical listing of all of the contents of a library, organized with a single card for each item in the library. It was a familiar navigational hazard and blessing in all libraries well through the late 20th century, when physical catalogs began to be displaced by computerized versions. Some libraries have kept them, often as sentimental mementos, and a few actively maintain their listings, although this is most common in small, remote libraries.
The need to catalog books in some way has been present since they were invented. A good catalog enables people to know which publications a library has and where to find them, and many contain additional information that could be assistance to scholars. Early library catalogs were kept on scrolls or in ledgers, and they were often printed and distributed so that distant scholars could know which books a library had.
The concept of the card catalog was introduced in the 1800s, and it was a great help to scholars. These catalogs can be configured in a number of ways, and their organization makes it easy to add or remove books, and to find particular ones. Every time a new book enters a library, a card is created for it, with information like the title, author's name, subject, and location of the book.
There are a number of ways to set up the listing. A dictionary catalog lists every single book in a library in alphabetical order, so in order to find a book, a patron must know what the title is. They can also be organized alphabetically by author's name, or by keyword or subject. It is also possible to find systematic catalogs, also called classified catalogs, that list the books according to the library's categorization system.
As anyone who has used a card catalog knows, it can be a challenge to navigate one until the user gets the hang of it, especially in a new library. Many patrons were forced to call on the librarian for help while navigating a difficult catalog, but once a person became acquainted with the system, the listing was a huge help, allowing patrons to quickly identify the books they wanted and find them on the shelves.
Computerized catalogs are the norm at most libraries today, because they have a number of obvious advantages over physical catalogs, including space efficiency, searchability with multiple parameters, and the ability to easily update it with realtime data.