Splines are ridges or the teeth[1][2][3] on a drive shaft that mesh with grooves in a mating piece and transfer torque to it, maintaining the angular correspondence between them.
For instance, a gear mounted on a shaft might use a male spline on the shaft that matches 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 can be supported in the pilot planetary gearbox bearing in the flywheel. An alternative to 
splines is definitely a keyway and crucial, though splines provide a longer fatigue life.[2]
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