Parallel Material Planes
Bringing the discussion back to multiple planes, I'm thinking of a campaign that involves multiple parallel prime material planes, sort of a cross between Moorcock's multiverse, H. P. Lovecraft's Dreamlands, and maybe a smidge of GURPS "Infinite Worlds". (Less "what if Queen Victoria died in childbirth", more "what if dinosaurs became the dominant sapients" or "what would a faerie realm look like".)
I'm looking for some weird physics to throw into the mix, for some of the more exotic worlds. Does the Manual of Planes cover constructing new planes, or is it all about the standard D&D planes?
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)
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