Choose Set Units from the Edit menu to select the units of measurement for your model. Set Units is available whenever a model is open. If no model is open, the command name changes to Set Default Units, and any changes you make to the dialog become the new default settings.
This command displays a dialog box which allows you to choose from pre-defined units of measure. Select Arbitrary to specify your own units of measure. You can also specify the abbreviation that's used for the units you define. In the dialog above, Ten Pixels is specified as the name for the arbitrary units, and each unit is equivalent to ten pixels.
The Snap subdivisions field allows you to specify the number of subdivisions within each unit. This value is then used when the Snap to Grid option is enabled. In the dialog above, each unit of ten pixels is divided into two subdivisions. Therefore, objects will snap at five-pixel increments. You can enter any integer value greater than zero in this field.
The Nudge subdivisions field allows you to indicate the number of subdivisions within each unit to use for nudging objects in your model. In the above example, each unit of ten pixels is divided into ten equal subdivisions. Therefore, each nudge is equivalent to one pixel. Enter any integer value greater than zero in this field.
Nudge works with any of the selection tools (Object Move, Rotate, or Scale tools). All nudge operations are relative to the active grid. The arrow keys nudge the selected object along the grid in the direction indicated by the arrow.
The Command key (Macintosh) or Ctrl key (Windows) used in addition to the Shift key and the Up or Down Arrows nudge the selection perpendicular to the active grid.
Remember, nudging occurs relative to the active grid. Therefore, if you're viewing the model from the backside of a grid, objects may appear to move in the opposite direction.
You can also specify how often grid lines are displayed. You can enter any number greater than zero in the Grid Lines field. In the previous example, grid lines appear every 30 pixels.
Models do not save the units in which they were constructed. If you change the default units, and then open a model that uses another scale, the current units and scale are adopted by the opened model.