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Notice cards are (were) 84 characters in width

Maybe I'm misunderstanding what you mean by a card, but the 7090 used 80-column punch cards of which 72 columns could be read into the computer. (Which is why Fortran used 72-character lines.) The 7090 had a plugboard so you could select which 72 columns you wanted, but normally it would be the first 72. Why 72? Because cards were read "sideways" and 72 columns fit into two 36-bit words.

Because cards were read "sideways"

That is, cards were read row by row, not column-(character)-by-column — one ‘bit’ from every character at once. And this is the case for practically all card equipment. That surprised me a bit when I first learned it, as someone used to sequential character streams, but it makes sense in the context of pre-computer card operations, where cards and holes started and stopped counting wheels for each digit (column) at once to calculate totals.

The IBM System/360 moved to card readers that operated column-by-column, e.g. the 2501 card reader or the 2560 Multi-Function Card Machine. As you said, reading row by row makes a whole lot of sense when you have electromechanical equipment. It's amazing that they could build things like card sorters without any electronics, just a brush and solenoids.

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