Two important concepts in gearing are pitch surface and pitch position. The pitch surface area of a gear is the imaginary toothless surface area that you would have got by averaging out the peaks and valleys of the individual teeth. The pitch surface area of an ordinary gear is the form of a beval gear china cylinder. The pitch angle of a gear is the angle between the face 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 since the gear teeth stage outward. The pitch areas of meshed exterior 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 which have pitch angles of greater than ninety degrees possess teeth that time inward and so are called internal bevel gears.
Bevel gears that have pitch angles of precisely 90 degrees possess teeth that point 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 the same numbers of teeth and with axes at right angles.
Skew bevel gears are those 
for which the corresponding crown gear has the teeth that are straight and oblique.