Edit Bar control: Ok
This tools adds an equal constraint between two similar entities.
To use the tool, identify either a pair of lines or a pair of arcs.
After creating the equal 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 them to include more entities. When you have finished identifying them, if they are lines they will be constrained to be all the same length, whereas if they are arcs they will be constrained to all have the same radius.
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.
Pre-selected Entities
Alternatively, you can select several entities (either all lines or all arcs) and then start the tool. The equal constraint will then be added to all selected entities.
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.