Two important ideas in gearing are pitch surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would have by beval gearbox averaging out the peaks and valleys of the individual teeth. The pitch surface area of an ordinary gear is the shape of a cylinder. The pitch angle of a equipment is the angle between the face of the pitch surface and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is called external because the gear teeth stage outward. The pitch surfaces of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of both surfaces are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees possess teeth that time inward and are called internal bevel gears.
Bevel gears which have pitch angles of exactly 90 degrees possess teeth that point outward parallel with the axis and resemble the points on a crown. That is why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equal amounts of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown equipment has the teeth that are straight and oblique.