Two important ideas in beval gearbox gearing are pitch surface area and pitch position. The pitch surface area of a gear may be the imaginary toothless surface area that you would have by averaging out the peaks and valleys of the average person teeth. The pitch surface area of a typical gear is the shape of a cylinder. The pitch angle of a gear is the angle between your face of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is called external since the gear teeth point outward. The pitch surfaces of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of the two surfaces are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles in excess of ninety degrees possess teeth that point inward and are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that point outward parallel with the axis and resemble the factors on a crown. That is why this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equivalent amounts of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear has teeth that are directly and oblique.