it's a coordinate. if I call you up and give you my pap,specs etc.. and say I want a 4 x 5 drilling. where do you place the pin?

the pin is the most important location. by telling you I want it 1" above midline then you know.

it removes a lot of confusion, say by call it rev leverage, where customers may not have the same lingo as there driller.
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"deserves got nothing to do with it."
-- William Munny