eli5: why do we know a note is off key even when we’ve never heard the piece of music before?

Question

Whenever I hear someone play an instrument, I can always tell when they’ve played the wrong note even if it’s my first time hearing it, why is that?

In: Other

Answer 1

Songs are written using scales. Which scale you are using is your Key. If someone plays or sings a note that doesn’t belong in that scale, it’s very easy to notice. Sometimes it can be intentional and even pleasant to play “wrong” notes or bend the key. But if it’s unintentional to play a note out of key, it’s generally going to be very noticeable and jarring since your ear is tuned to the specific key of the song subconsciously