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 equipment mounted on a shaft might use a man spline on the shaft that matches the female spline on the gear. The splines on the pictured drive shaft match with the female splines in the heart of the clutch plate, as the smooth tip of the axle is supported in the pilot bearing in the flywheel. An alternative to splines is usually a planetary gearbox keyway and important, though splines give a longer fatigue life.[2]