Home Run?
 

Batted ball hits the top of the fence, hits the foul pole, and then falls over the fence for a home run.

At what point, though, does it become a home run? When it hits the foul pole, or after it falls over the fence?

Generally the foul pole above the wall height is a home run, unless the ground rules indicate otherwise. It would then be a home run when it hit the foul pole.

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By rule hitting the foul pole above the wall height is a home run because it has left the playing field in flight and is a fair ball. Groundrules cannot change this.

Hitting the top of the fence in fair territory and bounding over the wall would make it a home run. Hitting the foul pole (with a normally located foul pole) might confirm it was a fair ball when it hit the top of the fence. There is a possibility that it was a foul ball depending on where it hit the top of the fence and the configuration of the field. PBUC/MLBUM does allow a ground rule to supersede their "top of the wall" interp. I can't picture how that would happen.

I can say with certainty that in this case the ball hit the top of the fence in fair territory, then hit the foul pole.

Probably one of those things I'll never see again.

Home run. EZ PZ.

BAS's question is not, was this a home run, but when did it become a home run.

First event, hit the top of the fence in fair territory. If the ball bounced back into the field, this would not be a home run, so hitting the top of the fence is not enough to call it a home run. If it bounces away from the field into DBT, then it is a home run after the bounce, when it leaves the playing field.

In BAS's case, the ball bounces parallel to the fence toward the fair pole. Unless the field geometry is weird, this call is either over the fence or over the field on it's way to the fair pole, it is not a home run yet. When the ball hits the fair pole above the top of the fence, it is now a home run and whether it now falls onto field or into DBT fair or foul does not matter.

I didn't see a parallel bounce in the OP but he might have meant that.