Pierre Maillard reviews the state of the touring bicycle in 1921. He styles himself ‘Ingénieur E.C.P.’ which might mean that he graduated in engineering from the prestigious École Centrale Paris, and he bears a surname that would become famous in french cycling circles.
He briefly comments on derailleur systems by Audouard, Boizot, Chemineau, Terrot and Whippet. He helpfully explains that the Boizot four and five speed systems include a mechanism for laterally moving the chainwheel to improve the chainline.
(Source gallica.bnf.fr/ Bibliothèque nationale de France)