Two important principles in gearing are pitch surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would have by averaging out the peaks and valleys of the average 
person teeth. The pitch surface of a typical gear is the shape of a cylinder. The pitch angle of a Gear rack equipment is the angle between the face of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch 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 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 possess teeth that point inward and so are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees possess teeth that point outward parallel with the axis and resemble the points on a crown. That’s why this kind of bevel gear is called 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 tooth that are directly and oblique.