Comment by amplitwist

Actually, this is a fairly unusual song, in music theoretical terms. Almost all pop songs use the I, ii, IV, V, and VI chords, with popular progressions including I-IV-V, ii-V-I, and I-V-VI-IV. Since this is tonal music, it obviously includes the I chord. It does use only four chords, but they are uncommon. Most of the song uses the I chord (G major in this case), but during the buildup, it uses an surprising I-II-III-I-II-vii progression. The II and III chords aren't strictly in the key of the rest of the song, although they still blend well with the in-key chords. vii is just F# minor here, but it is not in the key of the rest of the song either. vii is not often seen in pop/rock/classical music; you often see vii° instead, since it is in-key. I can't argue with Psy's choices, though, since that little riff builds a lot of tension.