Two important principles in gearing are pitch surface and pitch position. The pitch surface area 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 area of an ordinary gear is the form of a cylinder. The pitch angle of a equipment is the angle between the beval gearbox encounter of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This kind 
of bevel gear is called external because the gear teeth stage 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 of greater than ninety degrees have teeth that time inward and so are called internal bevel gears.
Bevel gears that have pitch angles of precisely 90 degrees possess teeth that time outward parallel with the axis and resemble the factors on a crown. That’s why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equal numbers of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown gear has tooth that are straight and oblique.