About the Corner Command
After clicking Corner , the possible corner options are available in the Tools Palette. By default, he Trim All Elements is selected.
Trim First Element: trims only the first line you selected to create your corner.
No Trim : no line is trimmed.
Standard Lines Trim : the trimmed lines are set as standard lines.
Construction Lines Trim : the trimmed lines are set as construction lines.
Construction Lines No Trim : the trimmed lines are set as non-trimmed construction lines.