There are numerous, slightly obscure, 19th century patents that can claim to describe derailleur systems , but three stand proud:
All three can claim to be real working systems, all three were produced commercially (at least to some extent), all three failed to catch the public’s imagination and none of the three can really properly claim to contain much of the design DNA of the modern derailleur. Perplexing.
Browse the derailleurs from before 1900...
Where you see a red link, this is for a derailleur model which I do not (yet?) have in my collection - but for which I do have some kind of real and relevant documentation.