Two important ideas in gearing are pitch surface area and pitch position. The pitch surface of a gear may be 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 beval gearbox equipment is the angle between the encounter of the pitch surface area and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This type of bevel gear is named external because the gear teeth point 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 in excess of ninety degrees possess teeth that point inward and are called internal bevel gears.
Bevel gears which have pitch angles of exactly 90 degrees have 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 numbers of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown equipment has tooth that are directly and oblique.