Now if you've played guitar for any length of time you may have come across those signs in guitar shops that explained that you would be politely required to not play some songs. For instance, it'd be common enough to get the friendly prohibition against Led Zeppelin's "Stairway to Heaven". In the Pacific Northwest an additional prohibition was leveled at "Smells Like Teen Spirit". It stuck with me a long time when someone explained that the Nirvana hit could be construed as Boston's "More Than a Feeling" in a minor key.
Well, anyway, "Stairway to Heaven" has been annoying guitarists in guitar shops for decades now and it makes me wonder whether we couldn't discuss a few elements about the song. That punning reference to Plant's line "and it makes me wonder" is an excuse to discuss what I'll call a i-IV alternating progression. If you're in A minor then this would come across as an A minor chord moving to a D major chord. In "Stairway" we could describe this as an A minor 7th chord going to a D major chord with a few decorative flourishes.
What's happening here? Melodically and textually Robert Plant is just doing his thing. Harmonically what we have is a chord progression that is just shifting back and forth between what can be called i and IV in the dorian mode.
So what is the dorian mode?
Well, famously, if you take all the white keys of the piano and play from D to the next D you get the dorian mode.
D E F G A B C D
The distances between the tones are in whole (W) steps and half (H) steps as follows
D E F G A B C D
W H W W H W
Now what you should be able to spot (if formatting doesn't betray me) is that there are half steps between E and F and B and C. The third scale degree is minor and the sixth scale degree is major while the seventh scale degree is minor. This can't be construed as a major or minor scale in common practice terms because there is no leading tone (as there is in harmonic minor). The reason this is significant in this case is that what we get is a symmetrical mode. The whole-tone scale is a symmetrical mode and the dorian mode is also a symmetrical mode, the intervals are the same whether we're going up or down through the length of the scale.
The dorian mode naturally creates a harmonically static and airless mood because of the symmetrical nature of the scale/mode. If you want an example of a stereotypically dorian mode solo the Doors song "Light My Fire" would be as stereotypically dorian as possible. For "Stairway to Heaven" that floating and weightless vibe fits as a backdrop over which Robert Plant can moan, "aawwwww and it makes me won-deeeeerrrrrrrrrrrrrrrrr" because the i-IV chord change brings in the dorian mode long enough to make a person feel like they're floating on some pot and LSD-induced magical cloud of wondering magicalness (or gearing up for more of Robert Plant's screeching by the end of the song).
You can hate the song all you want and it's not a personal favorite but as an explanation of why thee i-IV is so steady in rock thanks to the dorian implications of the harmony it's hard to find a better example.
Zeppelin wasn't the only band to exploit the floating dorian i-IV chord change, it's the magical floating chord change that opens up Pinkfloyd's masterful art-pop touchstone Dark Side of the Moon. If you're curious about how the dorian implications of i-IV can be heard as a structuring device in Dark Side of the Moon as a kind of musical triptych (but more in its current CD-era form than in its original LP playing form) .... .
If nothing else perhaps it'll just make you wonder how many songs use an alternation of a minor tonic and a major subdominant chord, that old A minor to D major switcharound.