Examining Jon O'Birs Ways And Means

Recently been examining what makes this song so great and came across something a bit odd.br
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First of all i believe the track to be in F# Major consisting of the pitches F♯, G♯, A♯, B, C♯, D♯, and E♯br
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The chord progression only plays the root and the fifths in this sequence:- F# + A# for 2bars then A# + C# for 1 bar then C# + C for 1 bar.br
then it repeats but this time on the 3rd bar it plays C# + D# instead of C# + C.br
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now if u haven’t noticed already C isn’t a part of the F# major scale.br
i am hoping someone out there can tell me what this is called musically? and why it actually works so well in this song? br
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