Two important ideas in gearing are pitch surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface area that you would have 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 gear is the angle between your face of the pitch surface area and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is called external because the gear teeth stage outward. The pitch areas of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two areas are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in excess of ninety degrees have teeth that point inward and are called beval gearbox internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees possess teeth that point outward parallel with the axis and resemble the points on a crown. That’s why this kind of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equal numbers of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown equipment has the teeth that are straight and oblique.