Two important concepts in gearing are pitch surface area and pitch angle. The pitch surface area of a gear may be 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 a typical gear is the form of a cylinder. The pitch angle of a equipment is the angle between the encounter of the pitch surface and the axis.
The most familiar types of bevel gears have pitch angles of significantly less than 90 degrees and they are cone-shaped. This type of bevel gear is named external since the gear teeth stage outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of both areas are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles in excess of ninety degrees possess teeth that time inward and so are called internal bevel gears.
Bevel gears which have pitch angles of precisely 90 degrees have teeth that point outward parallel with the axis and resemble the points on a crown. That is 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.