Two important concepts in gearing are pitch surface area and pitch angle. The pitch surface of a gear may be the imaginary toothless surface that you would beval gearbox possess by averaging out the peaks and valleys of the individual teeth. The pitch surface of a typical gear is the form of a cylinder. The pitch angle of a gear is the angle between your encounter 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 type of bevel gear is called external because the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the gear shafts; the apexes of both surfaces are at the idea of intersection of the shaft axes.
Bevel gears which 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 precisely 90 degrees possess teeth that point outward parallel with the axis and resemble the factors on a crown. That’s why this kind of bevel gear is called a crown 
gear.
Mitre gears are mating bevel gears with the same amounts of teeth and with axes at right angles.
Skew bevel gears are those that the corresponding crown gear has tooth that are straight and oblique.