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One of my biggest design pet peeves is people using the word "affordance" when they mean "signifier".
Most of what I know about design, I learned from reading books. I learned what "affordance" and "signifier" mean by reading Don Norman's book, "The Design of Everyday Things".
Don Norman introduced the concept of affordances to the world of design.
Here's how he describes it on his website:
The term was originally invented by the perceptual psychologist J. J. Gibson to refer to a relationship: the actions possible by a specific agent on a specific environment. To Gibson, affordances did not have to be perceivable or even knowable - they simply existed.
One of the reasons that people use "affordance" to mean "signifier" is that Don Norman didn't introduce the term "signifier" until later. The early editions of his book only talked about affordances.
An affordance is an action a person can take with an object.
What, then, is a signifier?
A signifier is a clue given to the user that they can take that action.
The reason that I have a pet peeve with people saying "affordance" when they mean "signifier" is that you can have one without the other.
Let's imagine two examples. First, an object that has an affordance, but no signifier:
You build a secret door, leading to a secret room. You don't give people any clues that the door exists, or that they can open it. There's no handle on the front of the door, the hinges aren't visible, and there's no gap above or below the door. It's not a different colour from the rest of the wall.
This is an object that has an affordance, because you can open it, but no signifier to let you know.
Second, something that has a signifier but no affordance:
You're a set designer for a stage play, and you make a wall look more authentic by putting a door on it. The door doesn't work, because it doesn't lead anywhere, but it looks like a door. It has a handle sticking out, and a frame around it, and it's a different colour from the rest of the wall.
This is an object that has signifiers, but no affordance. You cannot open it, despite all of the signifiers suggesting that you can.
Having two separate terms for these two separate concepts is important because many affordances in software do not have signifiers, and it's a usability issue.
For example, many actions on small screen devices rely on gestures, such as swiping a list item to the left to see a "Delete" option. Usually the interface doesn't give any clue that the user can do this. Users are not made aware that this affordance is there. They either need to be aware of it because of platform conventions, or need to learn it from someone else.
Designers are already adding lots of affordances to their interfaces. What they're missing is signifiers. Using one term for both concepts at once makes it harder to point out when affordances need matching signifiers.
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