Two important ideas in gearing are pitch surface area and pitch angle. The pitch surface area of a gear may be the imaginary toothless surface area that you would have got 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 your 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 they are cone-shaped. This type of bevel gear is named external because the gear teeth point outward. The pitch areas of meshed exterior 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 in excess of ninety degrees possess teeth that time inward and so are called internal bevel gears.
Bevel gears that 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 named a crown gear.
Mitre gears are mating bevel gears with equal amounts of teeth and with axes in right angles.
Skew bevel gears are those that the planetary gearbox corresponding crown gear has teeth that are directly and oblique.