The ruler overlays the viewport and allows you to get a sense of scale and rough measurement for your model. The Ruler measures in millimeters.
Toggle the Ruler On/Off
To toggle the Ruler on or off you will need to open the Settings window where you can see the option "Show Ruler Over Scene". Check or un-check the box to turn the Ruler on or off. If you check the ruler to be visible, this setting will stay for every nTop instance until you decide to change it.
Using the Ruler
To scale the ruler, zoom in and out in the viewport. The ruler will automatically adjust and scale accordingly.
When you first turn the Ruler on, the 0-position will be snapped to the left side. To adjust the ruler position, move the mouse pointer to the ruler and hold down the left mouse button. This will move the values along the ruler, allowing you to line up the 0-position to your desired location. You can also snap the 0-position to the left or right of the viewport. If you move the Ruler and close the program, when you open a new file, the Ruler will be in the same position that you left it at.