Derailleur history, according to Velo News:

https://www.velonews.com/the-drivetrain-wars

For those deeply interested, Frank Berto's book _The Dancing Chain_ has
much more details on the history until about 2000.

Then of course there are the hub gears, variable drives, gear boxes etc.
plus weird drive mechanisms such as reciprocating levers...


--
- Frank Krygowski