Louis-Francisque Prével d’Arlay was an early derailleur designer based in Lyon, France. In the style of John Fitzgerald Kennedy and Dominique Strauss-Kahn, Louis-Francisque Prével d’Arlay seemed to like to reduce his name to mere initials: P. d’A.

As far as I am aware he produced two different derailleur designs, both of which used a pulley cage with two pulley wheels and incorporated his patent design for the springs that maintained chain tension (see French patent # 382602 addition # 9,875).

The first design used a lever on the top tube of the bicycle frame that rotated a long rod that ran down to a derailleur mounted on the chainstay. The derailleur end of the rod was bent in a short ‘L’ shape. As the rod was rotated the right-angled section of the ‘L’ pushed the pulley cage in and out, changing the gear. This device was initially constructed by M. Prével d’Arlay, but later it was manufactured by an engineering company called Proust & Duvoy who were also based in Lyon. Frank Berto claims that it was supplied with a three speed freewheel with 18, 24 and 36 tooth cogs.

Paul de Vivie (Vélocio) writes in the T.C.F revue Mensuelle of March 1913 that this, rod operated, derailleur appeared in 1908. It can therefore claim to be one of the first designs that changed gear using the combination a static multiple freewheel and a moving pulley cage with two pulley wheels. These are essential features of all modern derailleur systems.

The second P. d’A design is more obscure - but was a later development that was cable operated. Frank Berto claims that this design is covered by a Belgian patent, but this has eluded me so far.

Frank Berto further claims that P. d’A derailleurs were produced up until the start of the First World War, that they were not a runaway commercial success, and that a disappointed Louis-Francisque Prével d’Arlay turned to archaeology and died in 1926. It’s not clear whether it was the disappointment or the archaeology that killed him.