OK, one quick story from today.

The pilot was explaining the cabin pressurization system to me.

He pointed to a dial with two hands in it. He explained that the little hand showed the p.s.i. of the compressor, and explained that that stood for pounds per square inch.

Then he said the larger hand showed p.s.i.d. which is pounds per square inch differential, and that was the difference between the cabin pressure and the outside pressure.

Finally he said there is also p.e.p.s.i. but didn't explain what that was, so I asked what that is. He said "Oh, it's a drink like Coca Cola."