Two important principles in gearing are pitch surface area and pitch position. The pitch surface of a gear is the imaginary toothless surface area that you would have by averaging out the peaks and valleys of the individual teeth. The pitch surface area of an ordinary gear is the form of a cylinder. The pitch angle of a gear is the angle between your 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 kind of bevel gear is called external since the gear teeth point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of both areas are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees possess teeth that point inward and so are 
called planetary gearbox internal bevel gears.
Bevel gears that have pitch angles of specifically 90 degrees have teeth that point outward parallel with the axis and resemble the factors on a crown. That’s why this kind of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equivalent amounts of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown equipment has tooth that are straight and oblique.