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alternatehistory.com
The idea here is to get an invention built and popularized earlier. Here's how it works:
An invention (year OTL)
A PoD, preferably late as possible to allow for...
Necessary intermediate discoveries for...
An earlier alternate year of invention
An (admittedly simplistic) example
Artificial Satellite (1957)
Pedro Paulet goes public with his liquid rocket in 1895
Use of launch rockets in WWI; necessary expansion of materials production and electric wiring etc.
1940