A card for the 1930 census with 12 rows and 24 columns, the same as Hollerith's cards for the 1890 census. Hollerith was originally inspired by the round holes punched in railroad tickets by conductors, so the first cards for tabulation had round holes. By 1930, IBM kicked off the unending pursuit of denser information storage by moving from 36 columns to 45 columns and, finally, to 80 columns with rectangular holes. But to avoid paying rent for IBM's "business machines" the Census Bureau developed their own in-house systems, which remained based on the 24-column, round-hole card.

Each location has a predefined meaning ("Sex" and "Color" are two of the categories). To fit all the data from the forms filled out by hand by census takers in the field, the mapping from form to card is somewhat complex—the meaning of some columns depends on what was punched in earlier columns, for example. Tabulating the census involved 12 passes through the tabulator for all the cards, sorting them in various ways and modifying the wiring of the machine for different results.