Symmetry Constraint

1. Select Drafting | Constraints | Distance > Symmetry.
2. Select a line or a point (A).
3. Select the second element: a line or a point (B).
4. Select the symmetry axis (C). The elements will be set symmetrical in relation to the axis.

Set two lines, points, or a line and a point symmetric in relation to a symmetry axis.

The elements are symmetrical. The line acts as the symmetry axis. The circle (B) moves.