With the dawning of the 20th century came the dawning of the rear derailleur as we know know it:

• Terrot introduced a double pulley cage that we might recognise today. More than this, they brought a depth of professional engineering excellence that a properly organised and financially successful company could offer. The derailleur matured from ‘interesting tinkerer’s contraption’ to ‘desirable consumer product’.
• Charles Boizot and Louis-Francisque Prével d’Arlay introduced the idea of using a pulley based derailleur to move sideways and shift the chain across a multiple freewheel.
• But perhaps most of all, Paul de Vivie, (Velocio) maintained a relentless PR campaign conducted through the pages of Le Cycliste and the TCF Revue Mensuelle, promoting the value of gear systems that offered many gears, a wide range of ratios and a reliable method of changing gear. In an age when many saw the single fixed as the only ‘gentlemanly’ transmission, the importance of his writings cannot be underestimated. By creating and feeding the demand for the multi-geared transmission Paul de Vivie made it almost inevitable that eager engineers would work on developing better systems and that even eagerer consumers would purchase the results of their efforts.

It’s worth noting that, while Britain was widely viewed as the epicentre of bicycle development at the time, every single system on this page is French. It’s not for nothing that the word ‘derailleur’ has two ‘L’s and a ‘U’.