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I assume it has to do with them being rectangular. And while technically you could have a rectangle that is rotated, I think it's also plausible to assume that any diagonals are considered as rhombuses (rhombi?)

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I would call 2486 and 2684 a square and, hence, a rectangle. I think the theorem holds for all 8 variants of that.

2486 is only a square/rectangle if the keys are exactly square. They're certainly not in the example image shown.

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