Two important ideas in gearing are pitch surface and pitch angle. The pitch surface of a gear may be the imaginary toothless surface area that you would possess by averaging out the peaks and valleys of the average person teeth. The pitch surface area of an ordinary gear is the form of a cylinder. The pitch angle of a gear is the angle between your face of the pitch surface area 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 point outward. The pitch areas of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of both beval gearbox surfaces are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in excess of ninety degrees possess teeth that point inward and are called internal bevel gears.
Bevel gears which have pitch angles of specifically 90 degrees have teeth that time outward parallel with the axis and resemble the factors on a crown. That's why this kind 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 teeth that are straight and oblique.