Two important ideas in gearing are pitch surface area and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would have got by averaging out the peaks and valleys of the average person teeth. The pitch surface of an ordinary gear is the shape of a cylinder. The pitch angle of a equipment is the angle between your face of the pitch surface and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and planetary gearbox therefore are cone-shaped. This kind of bevel gear is called external since the gear teeth point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of both areas are at the idea 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 that have pitch angles of exactly 90 degrees possess teeth that time 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 amounts 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.