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I have an Arduino Uno rev 3. Today i tried to mount a breadboard as a shield for my Arduino on top of it. I plugged in the pin headers i bought and was surprised that the breadboard won't fit. As you can see in the picture below the distance between Vin and A0 is exactly the size of a pin, but the distance between pin 7 and 8 is less than that and so the breadboard won't fit on this side. What's the reasoning behind this (in my eyes) bad design ?Arduino Uno rev3 with pin header

user5675428
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    It is a mistake that became a feature for polarizing the shields: http://forum.arduino.cc/index.php?topic=117441.0 – Dave X Dec 12 '16 at 21:42
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    Thank you. If that wouldn't be so sad it would be a joke... – user5675428 Dec 12 '16 at 21:57
  • http://forum.arduino.cc/index.php/topic,22737.0.html is the older discussion, with pics of early prototype boards. – Dave X Dec 13 '16 at 03:57
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    Arguably, if shields could be plugged in either way around (and the wrong way damaging the Arduino and/or the shield) then that could be called bad design. – Nick Gammon Dec 13 '16 at 04:42
  • @NickGammon That's the feature part. Maybe a better way to key a protoboard/shield would be to block a pin or two. I think I remember seeing a red plastic plug used to block/key a in the socket on a IDE disk drive connector. – Dave X Dec 14 '16 at 04:53
  • That is probably a better idea. Ah well, it's like the old joke: "There's no reason for it, it's just always been done that way!" – Nick Gammon Dec 14 '16 at 05:08

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It is a mistake that became a feature for polarizing the shields: forum.arduino.cc/index.php?topic=117441.0

In an 2008 discussion Massimo says:

"I made that mistake, when I made the first arduino board."

at http://forum.arduino.cc/index.php?topic=22737.msg171839#msg171839 with: enter image description here

Dave X
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