Two important concepts in gearing are pitch surface area and pitch angle. The pitch surface area of a gear is the imaginary toothless surface that you would have by averaging out the peaks and valleys of the average person teeth. The pitch surface of a typical gear is the shape of a cylinder. The pitch angle of a equipment is the angle between your beval gearbox encounter of the pitch surface area and the axis.
The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is named external because the gear teeth point outward. The pitch areas of meshed exterior bevel gears are coaxial with the gear shafts; the apexes of the two areas are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles of greater than ninety degrees have 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's why this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equal numbers of teeth and with axes 
at right angles.
Skew bevel gears are those that the corresponding crown gear has the teeth that are directly and oblique.