There exists a special kind of gear, called the bevel gear. Bevel gears have two shafts intersect with their splines shaped as cones. These kinds of bevel gears are often used on shafts and are spaced at 90 degrees from each other.
 

Types of bevel gears are differentiated by the geometry of the overall design.
 

Here are some of the most common ones in use today.
 

1. Straight Bevel Gears. These kinds of bevel gears have a cone shaped pitch surface. The gear teeth or splines are straight and tapered towards the apex.
    
2. Spiral Bevel Gears. These bevel gears have angled curved splines. The effect of this design is graduated and smoothed spline interlocking.
    
3. Zerol Bevel Gears. While this kind of bevel gear is very similar to a straight bevel gear, the main difference is that the teeth or splines are curved instead of conical. The ends of each spline is parallel to the axis while the middle of the spline is wrapped around the gear.
    
4. Hypoid Bevel Gears. These bevel gears are similar to the spiral bevel gear with the only difference being that the pitch surface is hyperbolic in shape instead of conical. This makes the design of the hypoid bevel gear very similar to a worm drive.
    

As can be seen, bevel gears come in different designs according to the geometry of the splines and the gear plane. The straight bevel gear is similar to the zerol bevel gear except for the shape of the teeth or spline. The hypoid bevel gear is similar to the bevel gear except for the pitch surface of the latter. Clearly, the adaptation of the geometric shape is dictated by what the bevel gear is needed for, and which its function should be within a machine.