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 this, 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 female spline on the apparatus. The splines on the pictured drive shaft match with the feminine splines in the heart of the clutch plate, as the smooth tip of the axle is usually backed in the pilot bearing in the flywheel. An alternative to splines is usually a keyway and crucial, planetary gearbox though splines give a longer fatigue life.[2]