Interpreting Punch Cards

For purposes of this description, we'll talk about IBM-style 80-column punch cards. There were other types, but the IBM type is the best-remembered. These were cards about 8 inches wide and a little less than 4 inches tall, made of stiff cardstock and printed with 80 columns of 12 rows each.

When “punched” by a punch table, a character was literally “punched” into the card by cutting small oblong square holes into the cards.

From Wikipedia:

This IBM card format, designed in 1928, had rectangular holes, 80 columns with 12 punch locations each, one character to each column. Card size was exactly 7-3/8 inch by 3-1/4 inch (187.325 by 82.55 mm). The cards were made of smooth stock, 0.007 inch (0.178 mm) thick. There are about 143 cards to the inch. In 1964, IBM changed from square to round corners. The lower ten positions represented (from top to bottom) the digits 0 through 9. The top two positions of a column were called zone punches, 12 (top) and 11. Originally only numeric information was coded, with 1 punch per column indicating the digit. Signs could be added to a field by overpunching the least significant digit with a zone punch: 12 for plus and 11 for minus. Zone punches had other uses in processing as well, such as indicating a master record. 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]). IBM and other manufacturers used many different 80-column card character codings. For some computer applications, binary formats were used, where each hole represented a single binary digit (or “bit”), every column (or row) was treated as a simple bitfield, and every combination of holes was permitted. For example, the 704/709/7090/7094 series scientific computers treated every row as two 36-bit words, usually in columns 1-72, ignoring the last 8 columns (the 72 columns used were selectable using a control panel). Other computers, such as the IBM 1130 or System/360, used every column. For operator and visitor amusement, in binary mode, cards could be punched where every possible punch position had a hole: these were called “lace cards” (such cards lacked structural strength and generally could not be further processed by unit record machines).

We also called those all-holes-punched cards “bugscreens” and a common way to annoy a fellow programmer (and the IBM service man) was to slip a bugscreen into the middle of a card deck. Generally, when this went through the sorter or the card reader, it would get hopelessly jammed around some part or another and require some time and effort to remove.