Splines are ridges or tooth[1][2][3] on a drive shaft that mesh with planetary gearbox grooves in a mating piece and transfer torque to this, maintaining the angular correspondence between them.
For instance, a equipment mounted on a shaft might use a man spline on the shaft that fits the feminine spline on the apparatus. The splines on the pictured drive shaft match with the female splines in the heart of the clutch plate, while the smooth tip of the axle is supported in the pilot bearing in the flywheel. An alternative to splines can be a 
keyway and key, though splines provide a longer fatigue life.[2]
