[link to original on tumblr]
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As far as I've figured out, it starts with a descending progression - Em D C B. And a similar sort of progression is at the very end - Em D C Em (with the second E minor lower than the first). What's interesting is that these two chord progressions both contain the same descending diatonic phrase (E D C B). In the opening progression, that phrase is the root of each of the chords. In the progression at the end, that descending phrase is still present, but the B note is present as the fifth in E minor (E, G, B) instead of the root in B major (B, D#, F#).
It's an interesting way to retain that phrase yet still return to the tonic in order to signal the end of the song.