Two important principles in gearing are pitch surface area and 
pitch angle. The pitch surface area of a gear is the imaginary toothless surface area that you would have by averaging out the peaks and valleys of the average person teeth. The pitch surface of an ordinary gear is the shape of a cylinder. The pitch angle of a equipment 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 they are cone-shaped. This type of bevel gear is called external because the gear teeth stage 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 that have pitch angles in excess of 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 equivalent numbers of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown equipment has tooth that are straight and planetary gearbox oblique.