Two important ideas in gearing are pitch surface area and pitch position. The pitch surface of a gear is the imaginary toothless surface that you would have by averaging out the peaks and valleys of the average person teeth. The pitch surface area of a typical gear is the shape of a 
cylinder. The pitch angle of a gear is the angle between the face of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is called external since the gear teeth stage outward. The pitch surfaces of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of the two areas are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in beval gear china excess of ninety degrees have teeth that time inward and are called internal bevel gears.
Bevel gears that have pitch angles of precisely 90 degrees have teeth that time outward parallel with the axis and resemble the points on a crown. That’s why this kind of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equivalent numbers of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear has tooth that are straight and oblique.