The key and lock is one of the oldest and best examples of a good interface. It’s guessable (there’s only one keyhole in the average door), secure (you have to know and have the right key, but it isn’t hard to use), consistent (know how to use one key, you know how to use almost all), and provides good feedback (key turns or it doesn’t).

Above the all the key/lock interface is good abstraction. Few people truly know how the tumblers in a lock work, and fewer still could design them. Yet the key abstracts the user away from the inner workings of the lock, allowing them to focus on the task (opening or locking the door) rather than how the tool helps them accomplish it.

Interfaces that have been around for centuries have done so for a reason, because they rely on UX truths that work no matter what the technology.

Guessability, security, consistency, feedback, and good abstractions are something that every interface (physical, software, or otherwise) should share.