The Undo option

It often happens that you inadvertently add objects which you don't want to a selection set during its compilation. When this occurs in the middle of a complicated selection it can be pretty annoying. Fortunately AutoCAD allows you to undo the last selection made during the compilation of a selection set. All you need do is enter U at the next "Select objects" prompt to remove the objects previously added.

Selecting All Objects

The All option is invoked by typing ALL at the "Select objects" prompt. You can use this option to select all the objects in the current drawing, no picking is required. Objects on Locked or Frozen layers are not selected but objects on layers which are simply turned off are selected.