Two important concepts in gearing are pitch surface and pitch angle. The pitch surface area 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 area and the axis.
The most familiar types of bevel gears have pitch planetary gearbox angles of significantly less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is called external because the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of both 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 point inward and 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 is why this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equivalent amounts of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown equipment has the teeth that are straight and oblique.
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