How does a Mechanical Clock Key get measured?

Question by many19: How does a Mechanical Clock Key get measured?
I have a few different clock keys and I also need to order some but I don't know what is the standard on how they measure the square hole diagonally or strain side to side, I'm useing my MM calipers to get the #.


Best answer:
Answer by Tech DudeI would measure across the flats (side to side).

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