Most users create their drawings in Model Space and use the sheets to setup the output of their drawings. The creation of geometry in the sheet mode is usually limited to inserting a sheet frame containing the drawing information, and perhaps also some text. Then, you setup your viewport(s), showing the geometry drawn in Model Space, or perhaps include also a viewport showing a particular detail of your drawing.

When setting up your viewport, in order to prevent accidental editing of the viewport/view setup, as best practice we recommend, once you have set the Viewport scale (e.g. Select the Viewport, then go to the Properties Palette, and under Miscellaneous>Standard Scale), that you lock the Viewport (Select the Viewport, then go to the Properties Palette, and under Miscellaneous> Display Locked).

Why is this useful?

If someone accidentally double-clicks inside the Viewport, and applies the commands Zoom or Pan, your Viewport will keep the view unchanged. If you don’t lock the Viewport, well, you are now using the Model Space, and loose the Viewport Scale you had set.

For a more extreme scenario, try this:

-          In a sheet, Zoom in to a Viewport (make sure is unlocked), enough so that you don’t see the Viewport lines (therefore you are unable to select them, or double-click outside to retur to the Sheet).  

-          Double-click with your mouse. Welcome to Model Space. You are now able to create/modify geometry, the lot.

So you have done all that you wanted, created/modified geometry, etc.…. But how do you leave the floating Model Mode, and return to the sheet? If the Viewport lines were visible, double-click outside the Viewport would return you to the SheetMode. If the Viewport was locked, you would just need to Zoom out, just enough to allow you to double-click outside the Viewport, and return to the SheetMode. Undo the actions until it returns you to a state/view where you are outside the viewport, is out of the question (after all, you just spent xyz minutes drawing and making changes to your drawing!). You are stuck in a viewport….

Well, not exactly: When working on a sheet, the SheetMode/ModelMode command lets you toggle from Model workspace to Sheet workspace. If you are “stuck” in a viewport, unable to click outside a viewport, in order to return to the sheet, Type  SheetMode, and hit enter. This command will deactivate the Viewport and you are back in/SheetMode.

To switch to Model workspace, use the ModelMode command.

Take a look at the video below for a demo of the scenario mentioned above.

SheetMode/ModelMode comand Demo