Extended Ortho Snap

Snap to points located orthogonal from existing points. Show Magnetic Point must be turned on.

1. Start a polyline with two linear segments, then start the third segment like this:
   
2. To end the third segment, first hover over the start point. This displays an auxiliary line extending vertically from that point. You can snap to points along this line.
   
3. Horizontal auxiliary lines can also extend from this point, when the cursor is to the right or left.
   

Note: If Always show auxiliary lines is checked in the Drawing Aids window, the auxiliary lines will be displayed, even when you move the cursor away from them.