Two important principles 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 possess by averaging out the peaks and valleys of the average person teeth. The pitch surface of an ordinary gear is the form of a cylinder. The pitch angle of a equipment is the angle between the face of the pitch surface and the axis.
The most familiar types of bevel 
gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is named external because the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the apparatus 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 possess teeth that point inward and so are called internal bevel gears.
Bevel gears which have pitch angles of precisely 90 degrees possess teeth that time outward parallel with the axis and resemble the factors on a crown. That is why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equivalent amounts of teeth and with axes at right angles.
Skew bevel gears are those that the corresponding crown gear has the teeth that are straight and oblique.