Rhythmic Displacement in Led Zeppelin, Part 5: 'Stairway to Heaven' (Again)

Last Sunday, I blogged about the rhythmic displacement of Robert Plant's 'When the Levee Breaks' vocals through Plant starting the fourth verse at the "wrong" time (a measure late, in that case). I concluded that blog by writing, "Since the underlying harmony throughout the verses is static, this rhythmic displacement does not cause any harmonic problems the way such a displacement would in, say, 'Babe I'm Gonna Leave You' or 'Stairway to Heaven' where the harmonies are more fluid." And yet I just discovered the other day that Plant does use that same "start at the wrong time" displacement technique in 'Stairway to Heaven' - not one measure late, like 'Levee', which certainly would cause clashes with the underlying harmony, but rather an entire phrase early (in this case, four measures).

The instrumental introduction establishes a four-square phrase pattern (four phrases, each four measures long). The first and second phrases are identical, so they're both labeled (a). And the third and fourth phrases are nearly identical, so they're labeled (b) and (b').

​0:00-0:54  Instrumental Introduction (16 measures)
   0:00   (a)   |a  E/g# |C/g  D/f# | FM7 |a |
   0:14   (a)   |a  E/g# |C/g  D/f# | FM7 |a | 
   0:27   (b)   |C D |FM7 a |C G |D | 
   0:40   (b')   |C D |FM7 a |C D |FM7 |

Plant's entry marks the start of the first verse at 0:54. That verse employs identical four-square harmony as the introduction.

The first phrase (0:54) corresponds to the lyrics "There's a lady...", the second phrase (1:07) to "When she gets..." Though the lyrics are different, the pitches and chords to which those lyrics are sung are essentially identical - the same melody and harmony are repeated, just with different words. That should come as no surprise as it is EXTREMELY common in all genres of pop music
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The third phrase (1:21), being harmonically different from the first two, has a correspondingly different sung melody (though the ending is quite similar).

The fourth phrase (1:34) is where things get really interesting. As was the case in the intro, the harmonies of the fourth phrase are comparable to the third phrase. Yet Plant's singing is comparable to the first and second phrases. This means that melodically the fourth phrase should be labeled (a), but harmonically it should be labeled (b'). In other words, there's a certain "divorce" between the melody and harmony. (And yes, I completely understand the connotations of that term in the context of popular music scholarship, and how I'm using it differently than other scholars.)

0:54-1:48  Verse 1 (16 measures)
   0:54   (a)   |a  E/g# |C/g  D/f# | FM7 |a |    “There's a lady...”
   1:07   (a)   |a  E/g# |C/g  D/f# | FM7 |a |    “When she gets...”
   1:21   (b)   |C D |FM7 a |C G |D |    “Ooo...”
   1:34   (a & b')   |C D |FM7 a |C D |FM7 |    “There's a sign...”

Plant "should" have repeated the tune of phrase three ("Ooo...") to match the harmony, saving "There's a sign..." for the start of the second verse (at 1:48). Instead, he displaces that entry by starting verse 2 a full phrase (four measures) early, overlapping with the concluding phrase of the first verse.