Using Layers to Organize Your Drawing - AutoCAD 2011 and AutoCAD LT 2011: No Experience Required - page 287

12. Start the Perpendicular osnap, place the cursor over the open, vertical
segment, and then click when the marker appears.
The horizontal line is extended to the location perpendicular to the
vertical line.
13. Select the grip at the open end of the vertical segment to make it hot;
then click the open end of the horizontal line to move the first end-
point there (see the bottom of Figure 6.42).
Place the cursor over this vertical line.
Pick this grip.
The Perpendicular osnap marker
FiGuRE 6.42 Using the grip to move the horizontal endpoint (above) and the vertical
endpoint (next page)