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To err is human.
Err, a word coined since the 14th century but used daily since humanity exists.
Senders and Moray (1991) established that we can have two different meanings for this word. First, we can assume err as a divergent action performed that should have been performed differently. Or, second, an action or event whose effects are completely different from the tolerances permitted.

The world of interaction controls/components has always existed since the machine is here. Although not in the way we know it today. In the beginning, a simple control has one meaning associated: intermediate the human will, the activation, and respective effect. The artifact was not equipped with much thought about the way it was aesthetically, how we should interact with it, and what would be the feedback, the reaction from it. 

By mid-1936, “Pilot error” was the designation of the major problems and causes for a handful of accidents in the American Air Force. Suddenly, thanks to Paul Fitts, some causes were defined as “designer error”. Thus the user-friendly world we know today was born.

Known in the UX world concerning the Fitts Law, Paul Fitts was hired by Air Force because he was a brilliant mind and was supposed to know how people thought. But his intellectual capacity allowed him to realize that understanding how the human being thinks, performs, and reacts to a situation is a huge and insane study and research of different subjects and domains. Everything has changed after Paul Fitts and Alphonse Chapanis realized that the B-17 Flying Fortress (one of the most used airplanes in that time) had the same toggle to retract the landing gear and to control the wing flaps. Two equal toggles positioned side by side. During at least two years this dilemma caused about 457 plane crashes.

B-17 Flying Fortress – the red highlight show us the two toggles. One to command the landing gear, the other to control the wing flaps.

Faced with so many accidents, meanwhile, Fitts declares a “designer error” instead of the typical and inconclusive “pilot error”. And from then until now the interactive world has changed. The focus passed to the capacity to understand every single touch, button, interaction and how can we create a good symbiosis between humans and machines. The “new” world of machines and Humans was full of unjustified and unfairly “Human errors” until then.

It was with Alphonse Chapanis that they proposed a solution to change the paradigm of ambiguous components for pilots. The idea was to implement a “shape-code”. The tenets of this were to create a wide range of buttons that at the end were distinct from each other, not only due to their visual shape but obviously and consequently also to the touch. A small step into the world of interaction that is reflected until these days. As I write this article, I feel that the space key is completely different from all character keys: and this allows me to realize that I did not make a mistake; it allows me to move from word to word with the certainty that makes me move forward. All components must be moved in their natural direction, Chapanis said, turning to the left means less or turn to the left. Rotating to the right means more or turn to the right. Toggle up, means ON (up, means on) and a toggle down, means OFF.

Creating “Psychologically Natural” interaction components, the machine has become more and more tailored to man, now also in a mental way.



  • Kuang, Cliff. USER FRIENDLY : How the Hidden Rules of Design Are Changing the Way We Live, Work, and Play. 2020.
  • Chapanis, Alphonse. “Psychology and the Instrument Panel.” Scientific American, vol. 188, no. 4, Apr. 1953, pp. 74–82, 10.1038/scientificamerican0453-74. Accessed 29 June 2020.

João Lima

→ UX Design Guru at Critical TechWorks - BMW Group → Founder → UX Teacher

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