As the name implies, index cards were widely used in the nineteenth and twentieth centuries to create an index to large collections of documents. A major law firm, for example, might have a room full of metal cabinets with drawers designed to hold index cards. Clerks might fill out several cards for an individual document or legal case, allowing them to be filed alphabetically under a number of terms.
One innovation based on the index card was the edge-notched card, which was an index card with prepunched holes near the edges. Users of this system assigned a category to each hole position, and then notched out the hole when a card fit a category. To locate all cards that matched a category, a long, thin rod or "needle" was inserted through the corresponding holes in a tray of cards, the cards were lifted out of the tray, and all of the cards with notched holes dropped out of the stack. The system could also be used to locate all the cards that belonged to one or more categories at once by using more than one needle.
While computers have largely supplanted index cards and especially edge-notched cards, index cards are still a popular way of organizing ideas, quotes, vocabulary words, and references while researching and writing books, articles, and term papers, and for managing any kind of free-form information (for example the Hipster PDA).
The 4 by 6 inch (102 by 152 mm) and A6 (105 by 148 mm) sizes are the only standard sizes that qualify for "postcard rate" when mailed through the United States Postal Service.