If no data was printed at the top of the card, the human would need to know what number represented and manually translate each column. If an error was noticed on the card, it would be re-printed. For these cards, you could examine the top of the card to see what was stored on the card. Most of the later punch cards printed at the top of the card what each card contained. In some cases, it wouldn't be possible to put the program back into order. If these cards were dropped or got out of order, it could take days or weeks to get the program back in order. One of the biggest fears of users dealing with punch cards was dropping the punch cards.
In the picture below, a woman stands next to the punch cards used in this program. The largest punch card program was from the 1950s SAGE air defense system, which used 62,500 punched cards (around 5 MB of data). If information was outputted (printed), it would be outputted as punch cards. After all cards were loaded into memory, the computer would be instructed to execute the code. As the reader read the information, it would be written to a computers memory. After the card reader has read a column, it moves to the next column. As the card is inserted, the punch card reader starts on the top-left side of the card, reading vertically from top to bottom.
To load the program or read punch card data, each card is inserted in a punch card reader to input data from the card into a computer. Because each card only holds so much data, if you write a program using punch cards (one card for each line of code), it requires a stack of punch cards. Once a card is completed, or the Return key is pressed, the card technically "stores" that information. Using a punch card machine like that shown in the picture above, data can be entered into the card by punching holes on each column, representing one character.
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