A major reason for the corner cut was so the punch card could not be inserted backwards or upside down. If the punch card was inserted backwards or upside down it hit a small plastic pin in the machine called the corner cut pin. This would engage a micro switch and halt the machine operation until the card was inserted properly with the corner cut on the correct side of the punch card as used in that system. Stopping the machine meant the machine would not continue to sort or validate.
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 were prepunched (e.g., a JCL command to start a new job) 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.
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.
A System 3 punch card.
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.
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 programable 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. 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 favor 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
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 was claimed by the Democratic Party to have influenced the 2000 U.S. presidential election in the state of Florida; critics claimed that voting machines which used punched cards to tabulate votes generated improperlyrendered records of several hundred votes, spread out over an entire state, which allegedly tipped the vote in favor of George W. Bush over Al Gore.
Some consider it to be a minor scandal that punch card-based voting machines have continued to be used over the next several years, including the 2004 U.S. presidential race. Others who have used the system for years without the slightest problem cannot understand how it could be such an issue.