tldr: SCOM starts in C# (with guitars tuned down a half-step, but you don’t need to). Starting chord progression of C# B F# is mixolydian, famous intro riff is C# major or mixolydian scale, doesn’t matter because it only uses notes they share. Chorus of G# B C# is still mixolydian, although the G# is altered from G#m to add a leading tone in the progression. First solo is in C# mixolydian, then repeat everything. Slash’s second solo begins at a key change to D#m, over the chords D#m B Bb G#m. The first two and last chords are from D# natural minor, over which Slash solos with the D# natural minor (aeolian) scale, but the third chord, Bb, is from the D# harmonic minor scale, creating the central point of contrast in the solo. Slash switches to the D# harmonic minor scale over the Bb chord each time through. He usually goes back to the aeolian over the G#m, but sticks with the harmonic minor scale over the G#m on the last time through, during his fast lead run. After that, the chord progression switches to D#m F# G# B C# F#, which is still in the D# minor key, except the G# is borrowed from the dorian mode to add a chromatic leading tone. Slashes switches to mostly the D# minor pentatonic scale on this part (with an F, the 2 note, sometimes mixed in). If you want to do your own minor scale lead here, use D# aeolian, but play the C note instead of B over the borrowed G# chord (equivalent to switching to the dorian mode scale momentarily).
Sweet Child O’ Mine
Sweet Child O’ Mine starts in C#/Db. The original is played on guitars tuned down a half-step to be able to use open strings, in which case you would play it as if it is in D, but you can also just play it in standard tuning a half-fret down. Or actually just play it in D. Just keep that in mind when you read a tab for it. I’m going to talk about it in terms of C# in standard tuning, so move any fret or note references up one if you are doing it one of the other ways.
The famous intro riff is at the high strings of the 11th through 14th frets. It uses parts of the C# major scale, with what you might call the third scale pattern (you might visually associate it with the dorian mode, but it would only be dorian if the key were different here). But then the rhythm joins in with a clear mixolydian progression, C# B F#, which is I bVII IV. You know instantly that it’s mixolydian because of the signature bVII chord that’s a full step down from the tonic (the B chord). See my Estimated Prophet post at the “California” paragraph for that explanation if you don’t already understand it. Anyway, the reason this wasn’t revealed during the opening riff is that the riff doesn’t contain the 7th degree of the scale that would let you know if it’s major (standard 7th, here the C note) or mixolydian (flat 7th, here the B note). In fact, the same riff would have worked over a major chord progression; it just would have been a different song going in a different direction.
So the verse follows that same chord pattern, with some arpeggios and flourishes off of the chords. The mixolydian scale works over this if you want to add more. Then the “Ohhh Sweet Child” chorus that goes V – bVII – I (G# B C#). The G# is not mixolydian – that would be G#m – because it has the standard 7th note in it. We aren’t really leaving the mixolydian theme, because the B is still there. We’re just using a chord from standard C# major here for the contrast and interest it adds, in that G#m has a B note as its minor third, and G# has a C note instead, and that C note leads your ear to the B chord in a progression more strongly than playing that same B note twice in two similar chords. If you were doing a lead over this other than the intro riff, you could momentarily emphasize the point of difference, the C note, over the G# chord before switching back to B and mixolydian over the others.
Then there’s a lead transition back to the main verse that uses the main verse chords with a distinctive solo in C# mixolydian. (note that the mixolydian flat 7th note only appears in the second half of the solo, creating another point of interest). Then we do it all again, and then….
The classic solo. Here, the song modulates (changes key) to D#/Eb minor, which is the opening chord of the progression underneath the solo. D# is the ii chord of the original C# key, and the modulation is smooth because the key of D# minor also has a major C# chord in it as its bVII chord. Even though the first two chords of the key are the same as the one we just left, they establish that D# is the new focus through the lead-up and emphasis, and then the subsequent chords. So, the new progression in this key is D#m B Bb G#m, which is a i VI V iv. One interesting thing is happening here. While the D#m, B, and G#m are standard for a minor key, the Bb (V chord) would normally be Bb minor in natural minor. Instead, the Bb major comes from the harmonic minor scale, which is the standard minor scale with a raised 7th note, here the D note.
The lead itself is mostly D# natural minor (aeolian) scale, but what defines the solo is that Slash switches to D# harmonic minor over that Bb chord. Over the next chord, G#m, he returns to the natural minor scale the first few times, but sticks with the harmonic minor when the chord is held for the fast lead run. This doesn’t clash because the notes of the G#m chord are in both scales (that it is, it doesn’t include the 7th degree note).
After that fast harmonic minor run, the underlying chord progression changes to D#m F# G# B C# F# repeated. This is the only reason you might want to lower your guitar tuning a half-step for the whole song, because here the D#m is on the low end, what would normally be an open E chord. And some of Slash’s solo is at the low end on the open strings too. You can play the D#m at the 6th fret in standard tuning it, it just doesn’t sound quite as good. Anyway, this progression is still in D#m, going i – III – IV – VI – bVII – III. The only notable chord here is the major IV chord, G#, which would normally be minor in a minor key, but is borrowed from G# dorian mode. What it’s doing here is serving a chromatic role in leading us through the progression. G#m actually works here and sounds fine, but G# with its C note sounds even better because it bridges a gap between the F# chord, which has a C# note in it, and the B chord which, obviously, has a B note it in . As I mentioned near the beginning when we used a G# instead of G#m for the same reason, the difference between those chords is the former has a C as its major third note and the latter has a B as a minor third. So focusing on only these notes, the choice is progressing down in half-steps with C# C B (using the G# chord that has the C) or C# B B (using the G#m that has a B). Again either chord actually works (try it), but using the major G# adds that extra pull towards the B with the leading tone nature of the C note that helps us step down from the C#.
If you are doing your own thing here on lead instead of playing Slash’s solo, use the D# aeolian, replacing the B with the C note over that G# chord (which is equivalent to using the dorian mode over that chord). But Slash uses the D#m pentatonic here (plus the major 2nd degree note, F), which avoids the issue because it doesn’t include either of the notes that make the difference between those two chords.
Finally there’s also a little pentatonic power chord run near the end mirrored by the same run on Slash’s lead – D#5 C#5 A#5 G#5 F#5 D#5.