To Apply a Parallel Constraint

Select the Constrain Elements tool.
In the tool settings window, set Method to Smart Constrain Elements by selecting the first icon.
Select the first line or ellipse.
Select the second line or ellipse.
The parallel constraint is displayed. An attempt is made to solve and remove one degree of freedom.

Note: To override or force a parallel constraint, set Method to Constrain Two Lines to be Parallel by selecting the fourth icon in the tool settings window.