If I use a 20 amp 250V plug in a 125V application, which post is neutral? Is it the vertical (like a normal 125V plug) or the horizontal?

If you are going to use a 250v "plug" incorrectly as a 125v then just make sure your prongs match the receptacle wiring. You really should use the correct parts for the job.

Thanks for your answer. I'm attaching a cord to a sprinkler pump (1.5HP) that is convertible between 125/250V. It's currently set to 250V and I will be using it that way. So for now, there is no "neutral". However, if I chose to change to 125V in the future (by changing a jumper within the pump), there will be a neutral. But you're quite right; in that case, I can wire the receptacle that I use then so the neutral is correct.

I was just wondering if there was a "standard" for which prong of this plug should be neutral.

Thanks again.

NO, there absolutely is NO "standard" because there is NO neutral in a 240v circuit.

DO NOT do what you are proposing. Either wire it for 240v, or 120v, and use the correct parts for each.

DO NOT incorrectly wire a 240v receptacle for 120v. Period.

If you use a 250 volt device for a 120 volt circuit, you will violate the UL listing, and the National Electric Code.

I'm properly admonished. I'll do it the proper way...

It's good to know this forum has a professional level of input.

KInd regards

Professionals? Where? Lasts weaks I cuden spells elektrision, now I ares one.