Helical and double helical gears
Helical gears, rather than the leading edges of the gear teeth running parallel to the axis of rotation, they run at a fixed angle. Thus, those gears look like a segment of a helix, or spiral. The angled gear teeth engage gradually, making helical gears run much smoother than traditional gears.
Citroën developed a double helical gear, resembling two single helical gears, threaded at opposite angles, stacked on a single axle. This allows for smoothness of helical gears, but adds the ability to take more torque than a traditional gear. Citroën returned to Paris and secured the patent for the double helical, or “herringbone" gear. Two herringbone “V"s, representative of the teeth of a double helical gear, still comprise the emblem of Citroën cars, and is recognized worldwide.
Citroën’s first factory, begun in 1904, mass produced double helical gears.