Two important ideas in gearing are pitch beval gearbox surface area and pitch angle. The pitch surface of a gear may be the imaginary toothless surface area 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 therefore are cone-shaped. This type of bevel gear 
is named external because the gear teeth point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two areas are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in excess of ninety degrees have teeth that point inward and so are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that time outward parallel with the axis and resemble the points on a crown. That’s why this type of bevel gear is called 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 that the corresponding crown equipment has tooth that are directly and oblique.
