yoke jaw engages a circular groove formed on the intermediate members. Both intermediate members are coupled together in turn by a swivel tongue and grooved joint. When the input and output shafts are inclined at some working angle to each other, the driving intermediate member accelerates and decelerates during each revolution. Since the central tongue and groove joint are a quarter of a revolution out of phase with the yoke jaws, the corresponding speed fluctuation of the driven intermediate and output jaw members exactly counteracts and neutralizes the speed variation of the input half member. Thus the output speed change is identical to that of the input drive, providing constant velocity rotation. Rzeppa joints A Rzeppa joint (invented by Alfred H. Rzeppa in 1926) consists of a spherical inner shell with 6 grooves in it and a similar enveloping outer shell. Each groove guides one ball. The input shaft fits in the centre of a large, steel, star-shaped "gear" that nests inside a circular cage. The cage is spherical but with ends open, and it typically has six openings around the perimeter. This cage and gear fit into a grooved cup that has a splined and threaded shaft attached to it. Six large steel balls sit inside the cup grooves and fit into the cage openings, nestled in the grooves of the star gear. The output shaft on the cup then runs through the wheel bearing and is secured by the axle nut. This joint can accommodate the large changes of angle when the front wheels are turned by the steering system; typical Rzeppa joints allow 45°–48° of articulation, while some can give 54°. At the "outboard" end of the driveshaft a slightly different unit is used. The end of the driveshaft is splined and fits into the outer "joint". It is typically held in place by a circlip. Birfield joints The Birfield joint is a type of constant-velocity joint based on the Rzeppa joint, but with the six balls confined using elliptical tracks rather than a cage. They have improved efficiency and are widely used in modern cars for the outboard driveshaft joints. The Birfield joint was developed by Birfield Industries and came into widespread use with the development of front-wheel drive cars such as the Mini. Tripod joints Tripod joints are used at the inboard end of car driveshafts. The joints were developed by Michel Orain, of Glaenzer