Index card

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An index card is a piece of heavy paper stock, cut to a standard size and often used for recording individual items of information that can then be easily rearranged and filed. The most common size in the United States and Russia is 3 inch by 5 inch (76 by 127 mm), hence the synonym 3-by-5 card. Other sizes widely available include 4 in by 6 in (102 by 152 mm), 5 in by 8 in (127 by 203 mm) and ISO-size A7 (74 mm by 105 mm). Cards are available ruled or blank, white or colored. Stationers sell special divider cards with protruding tabs and a variety of cases and trays to hold the cards.

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 two 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).