The gestalt principles or “laws” is a theory of human eagerness to see patterns by “making up” structures and relationships between indvididual parts – even when, strictly speaking – they are not there. Probably the most famous illustration of these principles is the Rubin Vase which can be viewed either as a vase or two faces depending on the viewer’s perspective.
The law of proximity
One of the gestalt principles is the law of proximity. That is, the notion that we perceive objects that are spatially close as belonging to the same group or entity.
When a principle such as this is not followed by designers, things can go terribly wrong. Or if you’re lucky, just annoyingly wrong. Let me give you an example.
A misleading airport sign
In an unfamiliar airport on my quest to finding the metro station I encountered this sign:

Or in a more readable version (the white and orange text is Danish and English respectively):

In my hurry I just quickly scanned the sign. And mentally I connected the Metro text with the arrow pointing down left and thus I went to the left.
As I didn’t find the metro station to the left I returned to the sign to take a picture (the blurry one above) and find out what had gone wrong.
The sign spurs a wrong mental mapping between text and arrow
Can you see the problem with this sign considering the law of proximity?
The designer intended to visually connect the Metro text with the arrow pointing up right, but got it wrong. The proximity of the Metro text and the leftmost arrow so-to-speak trumps the thin horizontal line separating the two disparate rows of information. This way the arrow to the far right is easily missed as in my case.
3 proposals for a sign redesign
So, how could the design of the sign have been improved? Without adding or removing elements but just moving things around lets have a look at 3 proposals.
Proposal 1

Proposal 2

Proposal 3

It’s hard to tell which is better (probably 2 or 3) but I bet that each of the three are superior in terms of usability compared to the original design.
Gestalt principles are useful in user interface design
This was an example of what can go wrong when gestalt principles are not followed on wayfinding signs at the airport. But the usefulness of these principles applies more broadly to user interface design – not least in the software world. For instance, the principles can serve as guide when judging the usability of design proposals and in understanding why test users may have problems navigating interfaces.