Edit Bar control: Ok
This tool adds an equal distance constraint between two pairs of nodes, where each node can be a point, line or arc.
To use the tool, identify a pair of nodes to define the first distance followed by another pair of nodes to define the second distance.
Each node can be a different type. For example, one distance could measure between two parallel lines and another distance could measure from the end point of a line to the edge of an arc.
When a distance to a line is calculated it always measures in a direction which is perpendicular to it. One consequence of this is that when both nodes in a pair are lines they need to be parallel in order to measure the distance between them. If the lines aren't already parallel when the equal distance constraint is applied then they will be made parallel. Therefore, applying an equal distance constraint to a pair of lines is like applying a parallel constraint at the same time.
When measuring the distance to an arc it can measure either to its closest edge or its furthest edge, depending on where the arc is identified.
Shift Modifier
You can also hold the Shift key down whilst identifying the nodes to include more pairs. When you have finished identifying them, all pairs of nodes will be constrained to be equidistant. After identifying two pairs of nodes the Ok button becomes active on the edit bar: 
You can click this button to finish the tool and add the constraint.
After creating the equal distance constraint you can hover your cursor over the constraint and all related entities and points will be highlighted.
NOTE: Whilst this tool can be used to move points and entities so as to satisfy the constraint, it is recommended that entities are created in the desired position.