Two important principles in gearing are pitch surface area and pitch angle. The pitch surface area of a gear may be the imaginary toothless surface area that you would have by averaging out the peaks and valleys of the individual teeth. The pitch surface area of an ordinary gear is the form of a cylinder. The pitch angle of a equipment is the angle between the encounter of the pitch surface area and the axis.
The most familiar kinds of bevel gears have pitch angles of significantly less than 90 degrees and they are cone-shaped. This type of bevel gear is beval gearbox called external since the gear teeth point outward. The pitch areas of meshed exterior bevel gears are coaxial with the gear shafts; the apexes of the two 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 time 
inward and are called internal bevel gears.
Bevel gears which have pitch angles of exactly 90 degrees possess teeth that point 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 the same numbers of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown gear has teeth that are straight and oblique.