Although I’ve known (and loved) the song most of my life, I just noticed the other day how clever the key change to the solo is in the Blue Öyster Cult’s “Don’t Fear the Reaper” – it goes from A minor to the chromatic mediant key of C minor using common pitches. The song is in an Aeolian A minor, with a pedal point on G natural (a minor 7th above the tonic – similar to the pedal on E in F# minor in “Wonderwall” a couple of decades later) throughout most of the song, but we get that one G# sounding like a leading note after the fermata Am chord just before the solo enters.
When the lead guitar enters, unaccompanied, playing arpeggios, this G# and the C (from the Am chord) are reinterpreted as the third (Ab) and 5th of an Fm chord in the first arpeggio (F Ab C) . Guitarist Buck Dharma then moves to a G7/F arpeggio (F-G-B) keeping his bass note (in the guitar line – there’s no electric bass yet) consistent – these become iv and V[4/2] in the new key. We then get a solo in a functional C minor when the full band re-enters, where leading notes are raised (i.e. using the harmonic minor scale). It ends with a sustained G natural (^5 in C minor) – the pedal point in the main A minor riff – as we return to the song.
It’s pretty tricky rhythmically as well – it all lines up in 4/4, or at least adds up to multiples of 4, if you count the arpeggios as eighth notes starting on 1, which means the cymbals enter on 1 after four bars and the full band enters on 2 after another four bars + one beat. The trick is that the arpeggio pattern is a three-note pattern accented in 4/4 so the barlines and chord changes always land in different spots in the pattern and it actually runs for a total of 33 eighth notes (eight bars + one beat). The drums are a bit unpredictable during the ensemble playing under the lead guitar but the bassist mostly keeps us anchored with the quarter-note pulse. (One can compare the intro of “Dust in the Wind”, where the melody line on the second string is a six-beat, three-note pattern C-B-D but the C-Am chord changes under this melody only occur every eight beats so the melody-harmony relationship is constantly changing.)