Routers typically comprise a baseplate of generally annular form from which pillars extend.  On the pillars is mounted a structure movable towards and away from the baseplate.  The structure includes a driving motor, usually an electric motor, on whose output shaft is mounted a collet for receiving a tool bit of a configuration appropriate to the task to be performed.  On moving the structure towards the baseplate, the tool bit moves through the central aperture in the baseplate and into engagement with the workpiece. 
The tool is fitted with a depth gauge that is set by a user and determines the extent of the downward movement of the structure.  The extent of the downward movement of the structure determines the extent of movement of the bit, which must be done accurately for accurate cutting. 
Typically, adjustment rings are provided which cooperate with a threaded section on the peripheral portion of the router motor assembly for motor assembly height adjustment.  Prior art height adjustment rings when used on the router in an upright position work quite well.  The height adjustment ring is used to raise motor assembly relative to the base and the force of gravity causes the motor assembly to be lowered relative to the base when a height adjustment ring is raised.  A problem occurs with conventional height adjustment ring routers when the router is used in an inverted position on a router table.  In the inverted position, gravity causes the motor assembly to move away from the base so an operator must exert an axial lifting force on the motor assembly in order to maintain the height adjustment ring in engagement with the base. 
Additionally, typical prior art routers include only a single upper zero reset ring for fine adjustment of the router bit.  With this configuration, when the router is in the inverted position on a router table or the like, the conventional upper zero reset ring is frequently obstructed from view.  This can be a significant problem when attempting to make accurate cuts.