To compensate for the cyclical nature of the Census Bureau's demand for his machines, Hollerith founded the Tabulating Machine Company (1896) which was one of three companies that merged to form IBM in 1911.
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 (0.178 mm) 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 or chips (in IBM usage). 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).
IBM punch card format
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 or 2 punches per column: digits (digit[0-9]) and signs (zone[12,11] - sometimes overpunching the Least Significant Digit). 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[2-4] + 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]). The punch cards were 7 and 3/8 inches long by 3 and 1/4 inches high and were 0.007 inch thick with one of the upper corners cut at an angle.
A major reason for the corner cut was so the punch card would not be inserted backwards or upside down. Operators loading card decks would quickly recognize an inverted card. Many computer installations used cards with the opposite corner cut (sometimes no corner cut) as "job separators", so that an operator could stack several job decks in the card reader at the same time and be able to quickly separate the decks manually when he removed them from the stacker. These cards (e.g., a JCL command to start a new job) were prepunched in large quantities in advance. This was especially useful when the main computer did not read the cards directly, but instead read their images from magnetic tape that was prepared offline by card to tape converters or smaller computers such as the IBM 1401.
It was common to have cards for particular applications pre-printed, with fields marked by vertical lines. Each programming language of the era had a special card form. Punch cards were used as legal documents, such as U.S. Government checks and savings bonds. For much of the 20th century IBM cards had the warning "Do not fold, spindle or mutilate," printed at one end, and that became something of a motto for the post-World War II era, though many people had no idea what spindle meant.
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. Specialized card punches could detect these marks and punch the corresponding information into the card. There were also needle 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.
Agway once issued debit cards that were punched instead of magnetically encoded.
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 (or row) 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, usually in columns 1-72, ignoring the last 8 columns (but this was programmable using a plugboard in the card reader and punch to select the 72 columns used). 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. Punch cards that held processing instructions were called control cards. 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 transaction. 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 favour in the mid to late 1970s, as disk storage became cost effective, and affordable 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.
Dimpled and hanging chads
One term for the punched card area which is removed during a punch is chad, although in IBM product nomenclature the term chip was invariably used.
Punch card based voting systems, the Votomatic system in particular, use special cards where each possible hole was pre scored, allowing perforations to be made by the voter pressing a stylus through a guide in the voting machine. These pre-perforated cards are called Port-A-Punch cards, a type introduced by IBM in 1958. One notorious problem with this system 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 was claimed by the Democratic Party to have influenced the 2000 U.S. presidential election in the state of Florida; critics claimed that punch card voting machines were primarily used in Democratic areas and that hundred of ballots were not read properly or disqualified due to incomplete punches, which allegedly tipped the vote in favour of George W. Bush over Al Gore.
Other punch card voting systems use a metal hole-punch mechanism that does not suffer nearly as much from this fault, although most states have eliminated punch card voting systems of all types after the 2000 Florida experience.