This is such an easy answer, I can't believe nobody has answered correctly.


You use a coarse thread bolt when the only nuts you have that match are
coarse thread.
You use fine thread bolts when the only nuts you have are fine thread.

You give up and head for the hardware store when one is metric and one is
standard and they are the only ones that will work for your project.


Eisboch (yeesh.... why is this so difficult?)