Add a Perpendicular Constraint

1. Select Sketch | Constraints | Add perpendicular constraint.
2. Select the first line. This line keeps its position.
3. Select the second line. This line moves so that it is perpendicular with the first line.
4. Add more perpendicular constraints by repeating steps 2 and 3 or finish adding perpendicular constraints by pressing the ESC key or the middle mouse button.

If you add a line perpendicular to another line by using the T key, a perpendicular constraint is automatically created.