Two important ideas in gearing are pitch surface 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 individual teeth. The pitch surface of an ordinary gear is the form of a cylinder. The pitch angle of a gear is the angle between the face of the pitch surface area and the axis.

The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This type of bevel gear is named external since the gear teeth stage outward. The pitch areas of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of both areas are at the point of intersection of the shaft axes.

Bevel gears which 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 exactly 90 degrees possess teeth that point outward parallel with the axis and resemble the planetary gearbox points on a crown. That is why this kind of bevel gear is called 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 equipment has the teeth that are directly and oblique.