Helical gears resemble spur gears with the teeth at an angle. They can be meshed in parallel or crossed orientations at 90 degrees or arbitrary angles and can be generated with as little as a single tooth forming a screw gear.
Gears may be specified in either the 'Normal' or the 'Radial' system or the fixed profile Sunderland standard, any of which can be generated as either Left or Right handed. Handedness in helical gears refers to the direction the teeth lean when the gear is placed flat on a table.
Using this add-in, proper Herringbone gears (such as the gears used for this add-in's thumbnail) can be created by using a Sunderland profile then mirroring the gear about one of its faces. In the case of Herringbone or other double-helix gears the handedness of the base gear is not as significant as it is for single helix gears - to effectively change the handedness of a Herringbone / double helix gear all you need to do is flip it over - whereas for a single helix gear it must be mirrored to change its handedness.
Finally, by setting a helix angle of 0 degrees, Spur gears can be created and defined in the metric system (as opposed to the sample Spur Gear script which defines gears in the American system) with this add-in.