Two important concepts in gearing are pitch surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would possess by averaging out the peaks and valleys of the individual teeth. 
The pitch surface of an ordinary gear is the shape 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 type of bevel gear is named external because the gear teeth point outward. The pitch surfaces of meshed exterior bevel gears are coaxial with the gear shafts; the apexes of the two 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 so are called internal bevel gears.
Bevel gears which 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 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 that the corresponding crown equipment has the teeth that are straight and beval gearbox oblique.
