The punch card (or "Hollerith" card) is a recording medium for holding information for use by automated data processing machines. Made of stiff cardboard, the punch card represents information by the presence or absence of holes in predefined positions on the card. In the first generation of computing, from the 1920s into the 1950s, punch cards were the primary medium for data storage and processing. They were an important medium, particularly for data input, well into the 1970s, but are now long obsolete outside of a few legacy systems and specialized applications.
The punched card predates computers considerably, having been originated by Joseph Jacquard in 1801 as a control device for the Jacquard looms. Such cards were also used as an input method for the primitive calculating machines of the late 19th century.
The version by Herman Hollerith, patented on June 8, 1887 and used with mechanical tabulating machines in the 1890 U.S. Census, was a piece of cardboard about 90 mm by 215 mm, with round holes. This was the same size as the dollar bill of the time, so that storage cabinets designed for money could be used for his cards. The early applications of punched cards all used specifically-designed card layouts. It wasn't until around 1928 that punched cards and machines were made "general purpose". In that year, punched cards were made a standard size, exactly 7-3/8 inch by 3-1/4 inch (187.325 by 82.55 mm), corresponding to the US currency of the day.
To compensate for the cyclical nature of the Census Bureau's demand for his machines, Hollerith founded the Tabulating Machine Company (1896) which was bought by Thomas J. Watson Sr., founder of IBM in 1914. IBM manufactured and marketed a wide variety of business machines and added the Hollerith card equipment to its line.
The IBM 80-column punching format, with rectangular, holes eventually won out over the Univac 90-character format, which used 45 columns (2 characters in each) of 12 round holes. IBM (Hollerith) punched cards are made of smooth stock, .007 of an inch thick. There are about 143 cards to the inch thickness; a group of such cards is called a deck. Punch cards were widely known as just IBM cards.
The method is quite simple: On a piece of light-weight cardboard, successive positions either have a hole punched through them or are left intact. The rectangular bits of paper punched out are called chads. Thus, each punch location on the card represents a single binary digit (or "bit"). Each column on the card contained several punch positions (multiple bits).
The IBM card format, which became standard, held 80 columns of 12 punch locations each, representing 80 characters. Originally only numeric information was coded with 1 punch per column (digit[0-9]). Later, codes were introduced for upper-case letters and special characters. A column with 2 punches (zone[12,11,0] + digit[1-9]) was a letter, 3 punches (zone[12,11,0] + digit[1-7] + 8) was a special character, The introduction of EBCDIC in 1964 allowed columns with as many as 6 punches (zones[12,11,0,8,9] + digit[1-7]).
Data was entered on a machine called a keypunch, which was like a large, very noisy typewriter. Often the text was also printed at the top of the card, allowing humans to read the text as well. This was done using a machine called an interpreter. Later model keypunches could do this as well. Multi-character data, such as words or large numbers, was stored in adjacent card columns known as fields. For applications in which accuracy was critical, the practice was to have two different operators key the same data, with the second using a card-verifier instead of a card-punch. Verified cards would be marked with a rounded notch on the right end. Failed cards would be replaced by a key punch operator. There was a great demand for key-punch operators, usually women, who worked full-time on key punch and verifier machines.
Electromechanical equipment (called unit record equipment) for punching, sorting, tabulating and printing the cards was manufactured. These machines allowed sophisticated data processing tasks to be accomplished long before computers were invented. The card readers used an electrical (metal brush) or, later, optical sensor to detect which positions on the card contained a hole. They had high-speed mechanical feeders to process around one hundred cards per minute. All processing was done with electromechanical counters and relays. The machines were programmed using wire patch panels.
Other coding schemes, sizes of card, and hole shapes were tried at various times. Mark sense cards had printed ovals that humans would fill in with a pencil. Specialied card punches could detect these marks and punch the corrisponding information into the card. There were also cards with all the punch positions perforated so data could be punched out manually, one hole at a time, with a device like a blunt pin with its wire bent into a finger-ring on the other end. In the early 1970s, IBM introduced a new, smaller, round-hole, 96-column card format along with the IBM System 3 computer.
Aperture cards are a specialized use of punch cards for storing "blueprints". A drawing is photographed onto 35 mm film and the image is mounted in a window on the right half of the punch card. Information about the drawing, e.g. the drawing number, is punched in the left half.
IBM punch cards could be used with early computers in a binary mode where every column was treated as a simple bitfield, and every combination of holes was permitted . In this binary mode, cards could be made in which every possible punch position had a hole: these were called "lace cards." For example, the IBM 700/7000 series scientific computers treated every row as two 36-bit words, in columns 1-72, ignoring the last 8 columns. Other computers, like the IBM 1130, used every possible hole.
In its earliest uses, the punch card was not just a data recording medium, but a controlling element of the data processing operation. Electrical pulses produced when the read brushes passed through holes punched in the cards directly triggered electro-mechanical counters, relays. and solenoids. Cards were inexpensive and provided a permanent record of each transactions. Large organizations had warehouses filled with punch card records.
One reason punch cards persisted into the early computer age was that an expensive computer was not required to encode information onto the cards. When the time came to transfer punch card information into the computer, the process could occur at very high speed, either by the computer itself or by a separate, smaller computer (e.g. an IBM 1401) that read the cards and wrote the data onto magnetic tapes or, later, on removable hard disks, that could then be mounted on the larger computer, thus making best use of expensive mainframe computer time.
Punched-card systems fell out of favor in the mid to late 1970s, as disk storage became cost effective, and interactive terminals meant that users could edit their work with the computer directly rather than requiring the intermediate step of the punched cards.
However, their influence lives on through many standard conventions and file formats. The terminals that replaced the punched cards displayed 80 columns of text, for compatibility with existing software. Many programs still operate on the convention of 80 text columns, although strict adherence to that is fading as newer systems employ graphical user interfaces with variable-width type fonts.
The term for the punched card area which is removed during a punch is chad. One notorious problem with a punched card system of tabulation is the incomplete punch; this can lead to a smaller hole than expected, or to a mere slit on the card, or to a mere dimple on the card. Thus a chad which is still attached to the card is a hanging chad. This technical problem actually influenced the 2000 U.S. presidential election; in the state of Florida, voting machines which used punched cards to tabulate votes generated improperly rendered records of several hundred votes, spread out over an entire state, which tipped the vote in favor of George W. Bush over Albert Gore, thus influencing history.
-  Doug Jones' Punch Card history site
- Various punched card codes
- The Undead - Wired magazine article about modern day use of punch cards
- A review of the book IBM and the Holocaust (ISBN 0609808990) by Edwin Black, an interesting take on IBM's involvement with Nazi Germany.