Quoting Sheldonbrown.com
- Older French bicycles used a 14 mm x 1.25 mm thread, but these are quite rare. French-threaded pedals are commonly labeled "D" and G" (French for "droite" and"gauche" (right and left). A French pedal will start to thread into a 9/16 x 20 crank (and vice versa), but will soon bind. Do not force it, or it will damage the crank. Aluminum French cranks are easily rethreaded to 9/16" x 20 TPI.
As opposed to R and L markings.