Two important ideas in gearing are pitch surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would planetary gearbox possess by averaging out the peaks and valleys of the average person teeth. The pitch surface area of a typical gear is the form of a cylinder. The pitch angle of a gear is the angle between the encounter of the pitch surface and the axis.
The most familiar types of bevel gears have pitch 
angles of significantly less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is named 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 point of intersection of the shaft axes.
Bevel gears which have pitch angles of greater than 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 time outward parallel with the axis and resemble the factors 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 gear has the teeth that are straight and oblique.