Splines are ridges or tooth[1][2][3] on a drive shaft that mesh with grooves in a planetary gearbox mating piece and transfer torque to this, maintaining the angular correspondence between them.

For instance, a gear 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 feminine splines in the heart of the clutch plate, while the smooth tip of the axle is certainly backed in the pilot bearing in the flywheel. An alternative to splines can be a keyway and key, though splines give a longer fatigue life.[2]