Two important principles in gearing are pitch surface area and pitch position. The pitch surface of a gear is the imaginary toothless surface area that you would have got by averaging out the peaks and valleys of the individual teeth. The pitch surface of an ordinary gear is the shape of a cylinder. The pitch angle of a 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 they are planetary gearbox cone-shaped. This type of bevel gear is called external because the gear teeth point outward. The pitch areas of meshed exterior bevel gears are coaxial with the gear shafts; the apexes of the two surfaces are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees possess teeth that time inward and are called internal bevel gears.
Bevel gears which have pitch angles of specifically 90 degrees possess teeth that point outward parallel with the axis and resemble the points on a crown. That’s why this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equivalent amounts of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear has tooth that are straight and oblique.