Two important principles in gearing are pitch surface area and pitch position. The pitch surface of a gear may be the imaginary toothless surface area that you would have 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 gear is the angle between the encounter of the pitch surface and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is named 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 which have pitch angles of greater than ninety degrees possess teeth that time inward and are called internal bevel gears.
Bevel gears that have pitch angles of precisely 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 numbers of teeth and with axes at right angles.