In Part 1 of this short introduction to jazz through Nat King Cole’s “The Christmas Song”, we introduced roman numeral annotation for chord progressions; how seventh chords form the basic harmonic unit in jazz; the walking I-IV (“one to four”) chord progression in the first line of the verse; and how the ii-V-I (“two-five-one”) turnaround can be used to glue chord changes together. Next, let’s expand upon these concepts to analyze what’s happening in the song.
Minor ii-V-I
When looking at the second line of the verse, we saw that although it would be perfectly logical for the chord progression to be a ii-V-I turnround to the IV chord (as demonstrated in this tutorial of the song), it actually doesn’t go to the IV at all. Rather, the progression goes to the vi (minor six, aka “submediant”), and the reason why this works is because the IV and vi triads share all but one note, therefore allowing them to substitute for each other despite having slightly different qualities (IV has a major quality, while vi has a minor quality).
But how should we then describe the chord progression towards the vi? Again, it’s nothing but the ii-V-I! Here the ii-V-I turnaround works more or less the same way for resolving to the vi as in our previous example of resolving to the IV. This time, we take B♭m7 as our temporary home key. The F7 serves as the dominant (V) chord to the B♭m7, and is used to create the climatic tension which demands resolution to B♭m7. The ii-V-I differs in minor keys from major keys in that the ii chord in the turnaround is typically a half-diminished seventh (sometimes annotated as iiø7), which consists of a diminished chord (1, ♭3, ♭5) with a minor seventh (♭7) instead of a diminished seventh (♭♭7), and the tonic chord which the turnaround resolves to is naturally minor (notated with lowercase i) instead of major (uppercase I). The ii, which would be Cø7 (C half diminished seventh, can be omitted or substituted with another chord. In this case, it is substituted by the E♭m7 chord. Here are the chords for the whole second line of the vese “Jack Frost nipping at your nose”:
Backdoor Progression
Let’s take a look at the ending of this phrase that transitions to the third line (“Yuletide carols…”) as there’s an interesting chord here, B7. In the key of D♭, B7 is a dominant chord built on the flat seventh scale degree, which is called the the "flat-seventh” chord and notated as ♭VII. In jazz, the flat-seventh chord is commonly used as part of what is known as the “backdoor progression”, which functions very similarly to the standard ii-V-I for resolving to the home key.
The “backdoor progression” can be written as iv7-♭VII7 (“minor four to flat seventh”) and starts with the minor four chord (G♭m7), going to the flat-seventh (B7) before resolving to the tonic (D♭Maj7). Just like in the ii-V-I, the chord leading up to the dominant (V) can be omitted or substituted - in this turnaround, the vi (B♭m7) chord is used instead of the iv7 (G♭m7).
Modal Mixture - “Borrowed Chords”
The use of the flat-seventh chord in the backdoor progression is an example of modal mixture, which is when a chord is borrowed from a parallel mode or key to provide extra color and harmonic variety. These chords are often referred to as “borrowed chords”. One very common example of a borrowed chord is the minor iv, or minor fourth chord. In a song that is set in a major key, the iv (or subdominant) consists of a major triad notated as IV. Using modal mixture, we can “borrow” the fourth chord from the parallel minor scale, and this often adds interesting tonal color to a piece of music. Take Radiohead’s “No Surprises” with its signature intro guitar riff, for example, which consists of a vamp between the I and borrowed iv chord:
To open this up a bit, let’s compare the chords on each scale degree of the minor key to that of the major key we covered last time:
1st – Tonic (key note) - i
2nd – Supertonic - iio
3rd – Mediant - III
4th – Subdominant - iv
5th – Dominant - V
6th – Submediant - VI
7th – Subtonic - VII
8th – Tonic (Octave) - i
In a minor key, the 7th chord is no longer the “leading tone” as the seventh scale degree is a whole step rather than a half step below the tonic, and hence it is referred to as the subtonic. (This is however different in the harmonic minor scale, where the seventh is the same as in the major scale and thus the chord is built on the leading tone - but let’s save that for another post).
As “The Christmas Song” is in D♭Major, the parallel minor key is D♭m, which has the following chords on each scale degree:
i: D♭m7
iio: E♭o7
III: EMaj7 (F♭Maj7)
iv: G♭m
V: Ab7
VI: AMaj7 (B♭♭Maj7)
VII. B7 (C♭7)
i: D♭m7
In the backdoor progression, we borrow the VII chord from the parallel minor key, which in this song means borrowing the B7 from the key of D♭m. Because the seventh scale degree is a half step lower in the parallel minor key than in the major key, when used as a borrowed chord we indicate that it is on the minor seventh scale degree rather than the major seventh and hence designate the term “flat seventh”, or ♭VII in roman numerals.
The following phrase of the song “Yuletide carols…” features another great example of a borrowed chord, G♭m6, which is the borrowed iv from the parallel minor key.
Modulation - “Key Changes”
What we covered so far - ii-V-I, the backdoor progression and modal mixture - provides basically all the ingredients needed to understand the myriad modulations that occur in the song. Modulation, sometimes referred to as “key change”, is when a piece of music shifts from one to another tonal center (i.e. tonic, or home key). Jazz music heavily features modulation, so much so that we often find modulation to multiple tonal centers within a single phrase of a melody - and that is exactly what we find in the verse of “The Christmas Song”.
In the line “folks dressed up like Eskimos”, you’ll notice a succession of chords that are outside of the key of D♭Maj7. What’s happening here is that the song modulates twice, first to the key of F Major, then to E major until it goes back to the home key of D♭ Major.
To understand how we managed to end up in the key of F Major, we need to look at the ending of the previous phrase, where we find a wonderful example combining the concepts we’ve covered so far. If we take the last two chords of the previous phrase, we can describe the chord progression (turnaround) to FMaj7 as follows (after “sung by a choir”):
If this looks familiar, it’s because it’s nothing but the ii-V-I turnaround - but with a twist! The Gø7 and C7 are the ii and V from a minor ii-V-I, and if you play those chords, you should hear the minor tonality. They sound like they should resolve to F Minor, right? Try playing this instead, and you should hear that it sounds quite natural:
So how should we understand this combination of the minor ii-V but resolving to FMaj7? You guessed it - modal mixture! What is happening here is that the ii-V-I turnaround is used to modulate from D♭ Major to a new tonal center of F Major, but the turnaround borrows the ii-V from F major’s parallel minor key, F minor.
This borrowed minor ii-V is always a delight to hear, and one great example can be found in the jazz standard “What Is This Thing Called Love?”. The song begins with chord changes that indicate a minor tonality, only to unexpectedly resolve to major I chord at the end of the phrase (“This funny thing called love…”) using the minor to major ii-V-I turnaround.
The rest of the phrase in “The Christmas” is more straightforward, using the standard ii-V-I turnaround first to E Major, then back to the tonic of the song’s key of D♭ Major. Here’s the ii-V-I on “…dressed up like Eskimos…”:
And on “…everybody knows…”:
Tritone Substitution
Finally, let’s look at how tritone substitutions are used to add variety and voice leading to ii-V-I progressions. The final phrase of the song (“…Merry Christmas to you.”) consists of a series of a series of diatonic ii-V-Is resolving to D♭Maj7. These are Fm7 and B♭7, followed by E♭m7 and A♭7 to D♭Maj7.
In my arrangement, however, you’ll notice that I’m using some different chords:
What I’m using here is a form of chord substitution called the tritone substitution, which involves exchanging a dominant seventh chord for the dominant seventh chord which is a tritone away from it. A tritone is an interval spanning three adjacent whole tones, or six semitones, and carries a strongly dissonant quality. As the note sitting between the fourth and fifth intervals, it is also referred to as the augmented fourth or diminished fifth. In a diminished triad, the tritone is the diminished fifth interval which provides the triad with its diminished quality.
To use tritone substitution for the B7 and A♭7 dominant chords in the last line of the song, we take the dominant chord a tritone above B, which is E7, and above A♭, which is D7. The result is a smooth, chromatic run from Fm7 back to the D♭Maj7:
Voice leading, which is the linear progression of the individual melodic lines which constitute chords (or harmony in general), is one reason why you might want to use tritone substitution. Tritone substitutions also add dissonance and tension that can help drive momentum towards resolution in a chord progression. As we explored in our discussion on the ii-V-I progression, the role of the dominant V is to create tension. Certain voices in the V, like the leading tone and the minor seventh, create a kind of gravity towards the I chord. The tritone substitution creates even further dissonance and tension, and opens up more options for how to resolve the tension.
Wrapping up
In this two-part series, we introduced key jazz concepts by breaking down one of the most iconic holiday songs of all time, Nat King Cole’s rendition of “The Christmas Song”. We looked at jazz harmony, the ii-V-I, modal mixture, modulation and the tritone substitution. Hopefully this has given you some insight into some of the elements of jazz music which give this song its charm.
As I was writing these articles, I realized that there has been a lot of information in this series which the reader might have found overwhelming or difficult to digest. Going forward, I’ll most likely change format in two directions: examine a song or artist on a more cursory, higher level while touching more lightly on music theory; narrow the scope to a particular part of a song so that we can deep dive into some particular aspect of music theory without overwhelming the reader with too much information.
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