Opening Poly Tool

To add an opening:

1. Click on the wall to locate the first corner of the opening.
2. Click several more times to place all of the other corners except one.
3. Double-click to add the last corner.

To edit the shape of the object, double-click on its outline. In the editing mode, you will be able to move, add or delete corners of the outline.

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Detailed info:
The 2D Elevation View