Have you ever approached a new door, inserted the key to open it, only to find that it wouldn't turn and the door wouldn’t budge? Then, upon turning it the other way, it opened without a hitch?
This micro-interaction - where you make a decision on which way to turn the key and are met with an error - is a prime example of poor User Experience.
A good user experience, naturally, would involve inserting the key, turning it effortlessly, and having the door open smoothly.
Why does this frustrating interaction occur?
It’s because you have a mental model for which direction the key should turn to open the door, yet this particular locking mechanism clashes with that model.
So how do we form that mental model?
After moving into a new place, it took me three weeks to consistently remember which way to turn the key to open the door. Even now, I find that muscle memory doesn't overcome the counter-intuitiveness of the mechanism. Because of this, I've given the UXability of door locks a lot of thought (definitely more than any reasonable person should) and come to some conclusions about the optimal direction a key should turn to open a door for an intuitive user experience.
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The short answer is that the direction the key turns, should match the direction the deadbolt moves.
[Keep in mind that clockwise = right, and counterclockwise = left]
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To explain this effectively, we need to do some basic terminology housekeeping.
Here's what you need to remember:
For simplicity, let's examine a left-opening door (the principle works in reverse for a right-opening door).