Here is a sandwich of three square sheet objects floating in space. (Yes that's a procedural space shader on my EnvSphere. No you can't find it online yet.) The three sheets are parallel, spreading out parallel to the XZ plane. Y is up as usual.

The bottom sheet is translucent green and passes very near but slighly below the origin of the world - point 0, -.001 inch, 0.

The top sheet is translucent pink and passes through the point 0, 5 inches, 0. Thus it is 5 inches away from (above) the green sheet.

The white sheet is positioned at 0, 0, 0, but currently I've applied a 3-inch displacement. Thus it is between the green and pink sheets. It's neutral position (without any displacement) would be 1/1000 of an inch above the green sheet.

The middle (currently white) sheet is going to be the subject of some experiments. The purpose of the pink and green sheets is to help us visualize the position of the middle sheet.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)