Two important concepts in gearing are pitch surface and pitch position. The pitch surface area 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 your 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 called external because the gear teeth point outward. The pitch areas 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 which have pitch angles in excess of ninety degrees have teeth that time inward and are called internal bevel gears.
Bevel gears which have pitch angles of specifically 90 degrees have teeth that point outward parallel with the axis and resemble the points on a crown. That’s why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equivalent numbers of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown reducers for greenhouse equipment 
has the teeth that are straight and oblique.