Parallel Constraint
Default UI Menu: Constraints/Parallel
Ribbon UI Menu:

Makes the first selected line parallel to the second selected line.
- Start with two lines. Activate the Parallel constraint and select the line you want to be made parallel.

- Select the line to which you want the first line to be parallel.

The lines are parallel, and the parallel constraint symbol is added to both lines.

- If you edit either line, the parallel constraint will be maintained.
