Smoothest key change when playing same song in different key?

Question

Let's say I'm playing a song. Let's keep this easy for this example "London Bridge" and I play it in C major. After I finish this song, if I'd like to play it again but in a different key, what key would that be. What would be the smoothest key change?

I'm currently thinking this would be G, as its the next key in the circle of fifths going clockwise. But is this right? Do I just go clockwise when I want to play the same song in a different key. Or is there a better way? (Maybe from C to G is not as interesting as other keys.. not sure).

Answer 1

Short answer: I think it depends on the melody and the chords, because key changes often sound seamless when the transition re-purposes the existing melody notes.

Long answer: key changes are often implemented using a transition/lead-in chord. For example, if you're transitioning from the key of Cmaj to the key of D♭maj, you might add a V chord (an A♭7 chord) before moving to the new D♭maj tonal center. The A♭7 would signify a coming key change and prepare the listener for the change.

I'll use London Bridge as an example. When played in the key of Cmaj, this song ends on the notes E-C (played over the word "la-dy" and over the chord Cmaj). But the notes E-C are also found in the chord C7, which can act as a V chord for a new tonal center of Fmaj. So you could modify the final chord progression in the following way:

Lyrics:              my    fair   la-dy   ...  London
Original chords:     G7     G7    Cmaj    ...   Cmaj
New chords:          G7     G7     C7     ...   Fmaj

And now the C7 chord is acting as a V chord for a new key center of F maj. In this case, you're moving up a fourth, and it would sound extremely natural--a good transition.

Alternatively, we could recognize that the final melodic notes E-C also appear in the chord Amin. So you could take the Amin and turn it into the ii chord of a ii-V-I leading to a new key center of Gmaj:

Lyrics:              my    fair    la-dy        ...  London
Original chords:     G7     G7     Cmaj         ...   Cmaj
New chords:          G7     G7     Amin    D7   ...   Gmaj

In addition to the melody, you can use harmonic patterns to modulate. For example, it's extremely common for modulations to move the tonal center up a half step. This can happen once or repeatedly, always modulating up a half step from the prior key. For example, if you're playing in the key of Cmaj, this would entail modulating up to the key of D♭maj. In the song London Bridge, this would work melodically too, because the V chord for D♭maj is A♭7, and the 3rd of an A♭7 chord is C--the same as the final melody note. Here's how it would look.

Lyrics:              my    fair    la-dy        ...  London
Original chords:     G7     G7     Cmaj         ...   Cmaj
New chords:          G7     G7     Cmaj    A♭7  ...   Dbmaj

As long as you can turn the melody into a common/shared tone between the old harmony and the new harmony, the transition will be smooth. Among these options, there is no objectively smoothest transition--it will be a matter of personal taste and familiarity. If you can keep the root the same and change only the quality (e.g., change the final chord from Cmaj to C7, with the idea of modulating up to Fmaj), then the transition will be even subtler. This tends to favor modulating up by fourths as opposed to modulating up by fifths.

Answer 2

Smoothest? Probably F. It's easy to 'fall into' the subdominant, with or without a C7 to grease the way. Most interesting? Well, the standard trick when you feel a tune has run out of interest in one key is to go up a semitone, to Db. Again, an Ab7 linking chord (doubtless using the C note as a pivot) is optional. Up a tone to D is also a standard trick. Or up a minor 3rd is a familiar and pleasant modulation, that's into Eb.

Will anyone be singing the song? If C is a comfortable key vocally, going up or down more than one or two steps may become uncomfortable.