Once upon a time, when the Arduino was still young, someone made a mistake with the headers on its PCB, placing one of the headers off the standard 0.1” placement grid - as used by just about everything else in the electronics prototyping world.
The result is history: Arduino shields always have one header on a 0.06” offset, instead of 0.1”. The big irony is that this mistake now benefits the Arduino world, since it uniquely differentiates it from everything else.
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