Using Layers to Organize Your Drawing - AutoCAD 2011 and AutoCAD LT 2011: No Experience Required - page 288

Select this grip first.
Click this grip to extend the vertical line.
14. Finally, with the polyline still selected, start the PEDIT command,
choose the Close option, and press ↵ to end the command.
Visually, there is no difference in the perimeter of the roof, but
AutoCAD no longer sees an open polyline. Closed polylines are almost
always preferable in case you need to extrude a 2D object into a 3D
object, and using closed polylines is generally a cleaner drafting prac-
tice. Your completed roof perimeter should look like Figure 6.43.
15. Save this drawing as 06A-FPLAY6.dwg .