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 that you would have by averaging out the peaks and valleys of the average person teeth. The pitch surface area of a typical gear is the shape 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 angles of significantly 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 areas of meshed external 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 so are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that time outward parallel with the axis and resemble the factors on a crown. That’s why this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equal numbers of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown equipment has teeth that are beval gearbox straight and oblique.