Two important principles in gearing are pitch surface area and pitch position. The pitch surface of a gear may be the imaginary toothless surface area that you would have got by averaging out the peaks and valleys 
of the individual teeth. The pitch surface area of a typical gear is the form of a cylinder. The pitch angle of a equipment is the angle between the encounter of the pitch surface and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is named external because the gear teeth stage 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 beval gearbox 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 which have pitch angles of specifically 90 degrees possess teeth that time outward parallel with the axis and resemble the points on a crown. That is 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 in right angles.
Skew bevel gears are those for which the corresponding crown equipment has tooth that are straight and oblique.