Two important concepts in gearing are pitch surface and pitch angle. 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 of an ordinary 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 kinds of bevel gears have pitch beval gear china angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is called external since the gear teeth stage outward. The pitch surfaces of meshed exterior bevel gears are coaxial with the gear shafts; the apexes of both areas are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees 
have teeth that point inward and are called internal bevel gears.
Bevel gears which have pitch angles of exactly 90 degrees possess teeth that time outward parallel with the axis and resemble the factors on a crown. That is why this kind of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with the same amounts of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown equipment has tooth that are directly and oblique.