Using Mirror with Sketch Constraints

With Constraint-based sketching active, sketch a line in the desired mirror position.
Use the right-click context menu, select Set as Mirror Line to set up the mirror.
With multiple Mirror Lines, entities are mirrored about each mirror line whether they are perpendicular or not.
To stop creating mirror constraints, uncheck Set as Mirror Line.

Note:

Splines are not supported with a Mirror Line, but the other Sketch tools are.
A Fixed constraint is automatically applied to the mirror line to ensure more predictable dragging. It is possible to delete this constraint but dragging behavior may deteriorate.
Equal Radius constraints are made automatically when creating multiple Rounded Corners with a Mirror Line.
To delete Mirror (Symmetry) constraints individually, click an entity to open the mini toolbar. Individual constraints can be removed using the Delete dropdown menu.

Note: 1D Patterns and 2D Patterns are not supported for Constraint-based sketching.