Splines are ridges or 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 fits the female 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 suggestion of the axle is usually supported in the pilot bearing in the flywheel. An planetary gearbox alternative to splines is usually a keyway and important, though splines give a longer fatigue life.[2]