Edit Bar control: Ok
This tool adds a symmetric constraint between two entities.
To use the tool, you first need to identify a line that will act as the plane of symmetry. Now identify a pair of entities (either two lines or two arcs). The entities will be constrained so that they are symmetrical about the first identified line.
After creating the symmetric constraint you can hover your cursor over the constraint and all related entities will be highlighted.
Shift Modifier
You can also hold the Shift key down whilst identifying successive pairs of entities to include more of them. When you have finished identifying them, all pairs of entities will be included in the constraint.
After identifying two entities whilst using the Shift modifier the Ok button becomes active on the edit bar: 
You can now click this button to finish the tool and apply the constraint.
NOTE: Whilst this tool can be used to move entities so as to satisfy the constraint, it is recommended that entities are created in the desired position.