This question regards the main theme from the video game The Last of Us. The video I will be referencing is someone playing it here:

There is a little introductory phrase at the beginning. Then, from 0:07 to 0:20, he plays the same riff three times, with little variations here and there. Then, on the fourth riff, the first note sounds off, but it still sounds good. The fourth variation of that riff, starting at 0:20, sounds somewhat eerie. Why is this? What is it about that note that makes it have that effect?

It subverts your expectations because, up until this point, that pitch hasn't been played. Your ears, attuned to Western tonality, assumed a minor key, and this particular pitch is not a part of that minor key.

The main riff is in E minor and uses the pitches G E A F♯ G. (The last pitch is harmonized with B in the second iteration of that riff.)

But the next pitch is A♯, the tritone of the key. This a dissonance with the E that plays as a pedal throughout, and it's also a dissonance with the overall key, because there is no A♯ in E minor.

One way we can conceptualize this is with the Implication-Realization Model. In short, there was an implication (sorry) of E minor; at the moment when the A♯ appears, the realization was not what you expected, which caused your response.

• Thanks, apologies if it was too basic of a question—I come from the land of CrossValidated and am not familiar with the rules here (I haven't really even played music seriously in years), but I felt like someone here would be able to explain why this subverted what I was expecting. Jan 16, 2018 at 1:37
• No worries at all! I changed the scale-degree stuff to actual pitches based on the key; I figured that would be more helpful. Jan 16, 2018 at 1:38
• No apologies, Mark, a good question. Jan 16, 2018 at 2:36
• You've called it A# in your answer. What are the reasons you prefer that to Bb?
– Tim
Nov 17, 2021 at 8:08

Adding to Richard's excellent answer, I guessed which note it might be before listening. It's what's become known as one of the 'Blue Notes'.

It's exactly half way between one root note and its octave. A tritone, as it's three tones away, either way. Not a part of E minor, diatonically, so it's going to catch one's ear as a 'wrong' note.

In blues, it gets called 'flat five' (b5), whilst in jazz it's sharp four (#4). As it comes right in between the other two stable notes in the scale - P4 and P5, both of which are totally non-dissonant (consonant), it's going to provide us with one part of music that's so important - tension. It's just wrong! And that's the point. One of my students describes it as 'sweet and sour'. Upon resolution, the world's o.k. again! It's in some ways like the semitone difference between notes 7 and 8 in a scale. 7 feels like it needs to go a little further to resolve. b5/#4 does the same, either down to a P4, or, perhaps more frequently, up to P5.

• I thought that beboppers tended to call it a 'flat five' instead of a 'sharp four'. Maybe that is the blues showing through in bebop....
– user39614
Jan 16, 2018 at 7:06
• @DavidBowling - always a difficult one to label, as P4 and P5 are there anyway, so which one gets changed into the tt...
– Tim
Jan 16, 2018 at 7:14
• @exnihilo It's flat five in both blues and jazz, including bebop; rarely, if ever, is it sharp four. Sharp eleven, yes, but not sharp four. Nov 17, 2021 at 4:24