Two important concepts in gearing are pitch surface area and pitch position. The pitch surface of a gear is the imaginary toothless surface area that you would possess by averaging out the peaks and valleys of the individual teeth. The pitch surface area of a typical gear is the shape of a cylinder. The pitch angle of a gear is the angle between the face of the pitch surface 
area and the axis.
The most familiar types of bevel gears have pitch angles of 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 external bevel gears are coaxial with the gear shafts; the apexes of the two areas are at the idea of intersection of the shaft axes.
Bevel gears which 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 have teeth that time outward parallel with the axis and beval gearbox resemble the factors on a crown. That’s why this kind of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equivalent numbers of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown gear has tooth that are straight and oblique.