Crown Gear
This gear is so called because the edges of it turn upwards like a crown. It is the same size as a standard sized 24-tooth gear, but its unusual teeth shape allow you to mesh it with other gears at right angles. The disadvantage of a gear like this is that is will provide a lot of friction and must be backed up against a beam or bushing to stop it slipping out of place. Despite this, it is still an important gear and very useful. It can be used in place of a normal 24-tooth gear but this is not advisable.

- The Crown gear meshed with a small, 8 toothed gear.

Bevel Gear
This gear is like the second incarnation of the Crown Gear, but it still has some differences. The first, and most important, is that it can only mesh with other bevel gears. It has 12 teeth and is more compact than the crown gear. It is only slightly more efficient, and can also be used in the Differential (three bevel gears are the only gears that fit inside it).

- Two bevel gears meshed at right angles.

Worm Gear
This gear is very unusual, but also very useful. For one full rotation of the worm gear, the gear that it is meshed with will move along one tooth. For example, the model on the left reduces the speed of the gear by 24:1 - a very compact and efficient way of gear reduction. Another bonus is that it is a one way function only, no amount of turning will make you turn it the other way (a useful feature in cranes, for example), although it will wiggle slightly as the size of the gear is just smaller than 2 large bushings. This wiggle can be eliminated by positioning the worm gear vertically in a structure.

- A small structure connecting a worm gear and 24-toothed gear

The Rack
The rack is a small, flat gear the size of a 4x1 brick. It can be used to convert gears into linear movement and is often used in car mechanisms. The rack must be able to slide freely, and there are generally three options for this. Up-turned beams (on the left), half beams (picture below) or smooth tiles. My technic Lego doesn't include any smooth tiles (and I cannot be bothered to get some from my other Lego to combine it) so I do not have an example of this. You must make sure that the cog is raised sufficiently from the edges of the rack otherwise the rack will not slide properly. That cog can be any sort of cog, but to make the structure more compact and to allow finer control, an 8-toothed cog is normally used.

It will not take much turning to get the rack out of the mechanism, so longer racks can be made by joining them together at the bottom.

- (above) A rack sliding on upturned beams

- A rack sliding on a yellow half beam (it that its name???)