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The most common punch card format was the IBM 80 column by 12 row, with narrow rectangular holes. However, there were other possibilities, such as a later IBM format that used round holes. That one was never widely used, but that's probably because it was introduced in 1969, at which point the days of punch cards were numbered anyway.

Intuitively it seems to me that round holes would be better from a mechanical stiffness viewpoint, making the cards less likely to jam in the reader. Is it the case that round holes would be mechanically better and the rectangular hole format was used because of inertia? Or did rectangular holes have some other offsetting advantage? Or did it just really not make any difference?

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    I think you need to justify why round vs rectangular holes really impact the mechanical stiffness. In my experience with punch cards (in the early 80's) the card stock was plenty stiff regardless of how many holes were in it. – Jon Custer Sep 9 at 14:42
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    "in 1969, at which point the days of punch cards were numbered anyway" - Thinks back to 1985 self, sorting through stack of fortran punch cards prior to submitting a job to the Sperry Univac 1100... – Glen Yates Sep 9 at 14:45
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    You shoulda bought a PDP-11 to use as an RJE station for that 1100 :-) – another-dave Sep 9 at 21:13
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    @JonCuster - the corner of the rectangle is a weak point at which a tear can start. That's probably not called 'stiffness' but still is a concern. (OT: fatigue failures in de Havilland Comet jet caused by rectangular windows, fixed by changing to ovals) – another-dave Sep 9 at 21:48
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    @another-dave - actually, that story on the Comet is incorrect - see sciencedirect.com/science/article/pii/S1350630797000058 is a good article - "All the observed cracks in the pressure cabin [1, 2] emanated from bolt or rivet holes near the cut-out areas. It was probably not the shape of the cut-outs that was so damaging to the fatigue life of the cabin, rather the method of fixing the windows and doubler plates onto the pressure cabin. Had the windows not been square then the "Redux" glueing method might have been applied to these areas, and the failure avoided." – Jon Custer Sep 9 at 22:10
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I'd have added this in a comment but don't have enough rep.

If you read far enough into the IBM history link given by another-dave in his answer, you'll find this quote that indicates the rectangular holes were in fact stronger:

As well as handling more data, the unique rectangular hole was stronger [emphasis mine] and more compatible with the wire brushes that electrically detected and gripped the holes, and thus was a patentable design. Such innovations inspired IBM’s effort toward further patents in tabluation—such as the Type 405 Alphabetic Accounting Machine and the 600 series of punched card machines—that came in a flurry during the 1930s.

So, at the least, the person who wrote that caption believed that the rectangular holes were stronger than the round ones.

This is the link to another-dave's original answer. https://retrocomputing.stackexchange.com/a/16119/19295

This is the link to the page with the above quote: https://www.ibm.com/ibm/history/ibm100/us/en/icons/punchcard/breakthroughs/

On that page, there is a set of four pictures. You need to push the > button to get to the one that has the quote (in the pic's caption).

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    Re, "the...hole was stronger." That's a hand-wave. A hole has no strength whatsoever. If somebody claimed that "X holes are "stronger" than Y holes," they actually are talking about the strength of something else—some aspect of the cards in which the holes are punched. But you can't describe the "strength" of a card with just one number. There's more than one measure of "strength," and that paragraph doesn't say what they actually measured. – Solomon Slow Sep 10 at 13:32
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    @SolomonSlow: It doesn't describe it, but knowing how IBM worked at the time, I think it's safe to guess that what they really measured was MTBF, which would basically work out to "how often did card readers jam". – Jerry Coffin Sep 10 at 17:22
  • @SolomonSlow It seems most understand that "strength of the holes" is a short-hand for discussing the reliability of the cards said holes are punched in. Indeed, this "whole" page is full of discussion on the "strength of the holes". As for the robustness of the answer I quote, yes, it obviously does not give a rigorous treatment of the subject. Sorry! I'll see if I can dig up some research on early 20th-century punch card construction and post back for you. – Loss Mentality Oct 16 at 14:44
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Round holes might have been 'stiffer', but rectangular holes won on packing density. When IBM invented the 80-column card (up from the previous 40-column Hollerith card of the same size), they determined you could get more columns per card by using rectangular holes.

IBM's own history describes two competing designs: the 80-column rectangular-hole card we ended up with, and a 45-column round-hole approach with more than one symbol per column. But it appears to have 'specialized' subcolumns:

thereby doubling the storage of data but with half of it devoted to alphanumeric characters.

There seemed to have been two important factors which resulted in the decision going to the rectangular camp:

  1. Tabulator operation would be simpler - I presume "one thing per column" being the factor here.

  2. Rectangular-hole card design could be, and was, patented.

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    True! But you could achieve the same result with round holes by including two characters per column like some of the other formats described on Wikipedia. Did the need for more holes that way, outweigh the better mechanical stiffness per hole? Or was it just that IBM defined their format first and everyone copied them? – rwallace Sep 9 at 12:18
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    Neither, at first - IBM defined the format, patented it, and thereby had an advantage in information density. Link. Eventually, of course, others copied. – another-dave Sep 9 at 12:26
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    @rwallace The 80-column, rectangular-hole card format is older than all-electronic computers. The codes had to be very simple in order for electro-mechanical sorting, tablulating, and printing machines to be able to work with them. – Solomon Slow Sep 9 at 12:28
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    P.S., Electro-mechanical sorters still saw wide-spread use right up to the very end of the punched card era—decades after all of the other equipment had been fully computerized. Those sorters simply could not have worked with compact binary encodings. – Solomon Slow Sep 9 at 12:43
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    Would using oval or rounded-rectangle holes have had any negative impact on density? IBM Cards which have too many holes punched in close proximity can be fragile, but would probably be less so if the corners of the holes were even slightly rounded. – supercat Sep 9 at 15:57
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However, there were other possibilities, such as a later IBM format that used round holes.

Not only later, but also previous IBM formats used round holes. Similar next to all other contemporary (1930s) manufacturers (Powell, CDC, Honeywell, etc).

Intuitively it seems to me that round holes would be better from a mechanical stiffness viewpoint, making the cards less likely to jam in the reader. Is it the case that round holes would be mechanically better

Yes. Round holes are less prone to ripping and keep the cards more mechanical stable.

and the rectangular hole format was used because of inertia?

Nope.

Or did rectangular holes have some other offsetting advantage?

It's called using patents to fight off competition. The rectangular holes were part of IBM's patent application for the 80 column card and the essential claims of 'newness'. Patent law requires an invention to be new, not better.

IBM's approach was to make their own equipment based on rectangular holes and fencing off competition. The fact that competitors did enable their machinery to read square and round holes was even in favour for IBM, as the market for (key) punches was way larger than for other equipment (sorters etc.). So only IBM punches could deliver cards for IBM machinery - and other manufacturers machinery as well.

Unlike often cited, there is no advantage in density for rectangular holes over round ones, as readability (movement for brush wires) is defined by the width of a hole (in reading direction) which is the same for round or square holeswhen packing 80 columns on a card. Making them 'higher' has no advantage as already early 1900s equipment did transport cards fine enough to stay within a few mil.

Or did it just really not make any difference?

From an engineers point of view it was worse, but it wasn't about engineering but IP.

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If you punched a wrong character with a key punch - (I have used them...), You could push a rectangular "chad" into a hole (with a bit of spit) and it would be retained sufficiently well to make it through a 600 card/minute card reader.

Would this be true for a circular cut-out?

BTW This was on ICL (ICT) 1900 machines, so I don't think it was a patent issue.

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    I don't think spit was patented :-) but the card format definitely was. Here is the patent. It might have resulted in vendors of cards rather than equipment having to pay a licensing fee to IBM. (Btw, I have also spat at ICL cards) – another-dave Sep 9 at 21:57
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    I wonder if these new-fangled round holes might be more prone to tearing, if a series of holes should line up in a row? I'm surprised that no reference was made to the stress concentrations present at the corners of rectangular cut-outs? – Jeremy Boden Sep 9 at 23:07
  • Research "hanging chads" in the aftermath of the Bush-Gore contest of the year 2000. – Walter Mitty Sep 10 at 12:52
  • From my site profile: Old enough to know that 'chad' is not just an African country. – another-dave Sep 10 at 14:56

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