Toolbar: 
Constraints toolbar 

Menu: 
Add menu > Geometry submenu> > Constraints submenu > Add Perpendicular 

What to do: 
Click Add Perpendicular Constraint, then click:
 or 
 or 


Result: 
 or 


Notes: 
Other terms for 'Perpendicular to...' are: 'Normal to...', 'RightAngles to...'. 
Before 
After 
Select: 
SelectionWindow GeometryTree 


The Lines becomes Perpendicular 
Perpendicular 

The Line becomes perpendicular to the X–axis 
Perpendicular 

The Line becomes perpendicular to the Y–axis 
Perpendicular 