This topic fascinates me but I wish I knew more about the theoretical aspects of harmony. John generously over-estimates my ability.
However, I am sure you would agree there are some songs that - when you first hear them - sound as though something weird or wrong has happened, usually at about the midway point of the chorus. You listen to them time and again until you discover the tune has embedded itself in your consciousness and the ‘weirdness’ begins to sound right. You then realise it’s the weirdness that gives the tune its memorable character.
Examples of tunes fitting this description (and I would be grateful if you would suggest any more) are When It’s Sleepy Time Down South, Do You Know What It Means To Miss New Orleans?, China Boy, A Nightingale Sang in Berkeley Square, Body And Soul, I’m Getting Sentimental Over You, Smoke Gets In Your Eyes and Tea For Two. They all take us by surprise with the chords they use in the Middle Eight.
What seems to happen in these 32-bar tunes is: (1) we begin with two similar 8-bar phrases in the home key; then (2) in Bar 17 we switch into a different key before (3) finding the dominant 7th in Bar 24 and so (4) returning to a final eight comfortably back in the home key.
As an example, consider When It’s Sleepy Time Down South. In the Key of F, its first two 8-bar phrases are not entirely orthodox. They begin on the chord of Bb major; but this is immediately followed by Bb minor, which takes us naturally into F in the third bar; and so we are comfortably rooted in F up to the end of Bar 16.
Then wow! We are suddenly in the Key of A, with the melody twice climbing the stairs and pottering around at the top of the stave.
But in Bar 24, we land on the A chord, which quickly transmutes into A7th sliding up to C7th – and so we are beautifully steered back into the Key of F for the reassuring final eight bars.
That is pretty much what happens in all these tunes. Probably the simplest is China Boy. It’s usually played in the Key of F but the Middle 8 is distinctly in Ab, with Bars 23 and 24 sliding us back into the comfort zone, via the chords of Ab and C7th respectively.
Then of course there is I Love Paris by the great Cole Porter. The chorus has 16 bars in the minor key followed by virtually the same melody in the major (achieving a sudden brightness from Bar 17). The first sixteen bars could be in C minor, the next sixteen in C major.
I may be wrong, but I think the proportion of classic songs using the harmonic key-changing trick is probably fewer than 10%. But they form an interesting group – offering so much more of a challenge to the musician and improviser than the hundreds of tunes with orthodox harmonic patterns.
