Two important ideas in gearing are pitch surface area and pitch position. The pitch surface area of a gear is the imaginary toothless surface that you would have 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 gear is the angle between the face of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch angles of significantly less than 
90 degrees and they are cone-shaped. This type of bevel gear is beval gearbox called external since the gear teeth stage outward. The pitch surfaces 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 in excess of ninety degrees have teeth that time inward and are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that point outward parallel with the axis and resemble the points on a crown. That’s why this kind of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with the same amounts of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown equipment has tooth that are directly and oblique.