Edit Bar control: Ok
This tool adds a horizontal constraint either to a single entity that has a direction or to two or more points.
To use the tool, identify an entity or pair of points.
If the constraint is applied to a single entity, such as a line or graphic entity, it constrains it to be horizontal. If the constraint is applied to two or more points it constrains them to align along an imaginary horizontal axis.
Shift Modifier
You can also hold the Shift key down whilst identifying them to include more entities or points.
After identifying a single line or two points 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 one or more entities and then start the tool. A horizontal constraint will then be added to each of the selected entities.
After creating the horizontal constraint you can hover your cursor over the constraint and the related entity or points will be highlighted.
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.