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By "sudden", I mean that there is no dedicated harmonic preparation leading into the new key.

The commonly used ones I can think of are:

  • Up a semitone (the classic signal that this is final repeat of the chorus before the fade-out in a pop or rock song): E->F
  • Make the dominant the new tonic: C->G
  • Make the subdominant the new tonic: E->A
  • Down a semitone (much less common than up a semitone): C->B

What other intervals between keys are often used for sudden and surprise key changes?

4

It depends on what genre you're looking at, but here are a few other quick ways:

  • Down a minor third. This is often done by way of a "common tone modulation," where a pitch (or two) are held over into the next chord. Imagine you're in C major; hold onto C (and E as well, if you want), but then slap an A in the bass. Suddenly you have an A minor chord (down a minor third from C major) and you can proceed in A minor. But this sounds a little too friendly sometimes, so consider...
  • Down a major third. Do it the same way: hold onto C (E won't work here, but E-flat would) and throw an A-flat in the bass, completing the A-flat major chord in the rest of the voices. This is very common in movie soundtracks today; it has a nice "epic" and "dark" feel to it.
  • Up a minor third. Again, let's say we're in C; we want to get to E-flat. From a C chord, just go to the dominant of E-flat (B-flat). This creates something like a predominant--dominant--tonic cadence in a new key (E-flat) with only one chord in between; it's very smooth and often has an "uplifting" feel that isn't quite as cliché as the ascending half-step modulation.
  • Up a major third. For this one, you don't need any intervening chords! You're in C major, and you end a phrase on the dominant (G). Well, instead of resolving to C, just go ahead and resolve to E major! It's a common enough modulation, but I always think of Strauss's Elektra, where he uses this to great effect.

Lastly, you mentioned that moving down a semitone is not as common as moving up a semitone; you're absolutely right! But it's almost just as easy to do it successfully. Imagine, for one last time, that you're in C; the V7 chord is G--B--D--F. Well, that F can also function as an E-sharp, in which case G--B--D--E# is a fancy chord (a German augmented sixth) that can lead right to the dominant built a half-step lower (F#). Long story short, you can play any dominant chord and then follow it with a dominant chord a half-step lower to lead to the new tonic. There are some specific part-writing rules you'd want to acknowledge depending on your genre, but play around with it and see how you like it! Play around in C, and reach a G7 chord...but then follow it with an F#7, and then resolve that to B. Super easy!

  • That last bit is sort of tritone substitution in a weird way! – Tim Jul 28 '16 at 11:36
  • Both concepts fall into the general category of "enharmonic reinterpretation" in some way. Also, the German augmented sixth is just the tritone substitution of V7/V! – Richard Jul 28 '16 at 11:38
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Going up a tone rather than a semitone works just as well. A wrenching one Ive played in some numbers is a minor third, straight in. Not too easy to pitch if the vocals lead it. Rarely played anything that dropped a semitone. It sounds quite odd, almost going downhill in feel.

Playing in bands that may segue several numbers together, and not necessarily knowing what the next key is until a couple of bars before, there has to be a couple of bars of some chord changes to get there,so a sudden change didn't used to work most of the time. Although some vocalists were quite adept at knowing the previous key (sung by another singer), and their next key, and could segue straight in, thus a sudden change as you say. From anything to anything. Got a few looks from some listeners, though.

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Country musicians often do a whole tone up change as in "Good Hearted Woman." It's mostly a "color" effect rather than a harmonic change. Many renditions of "Fever" use a sequence of whole tone up changes; this becomes a structural (but not harmonically functional based) part of the music.

Classically (and with some popular musicians) jumping either a major or minor third (up or down) has been used. This is rather abrupt sounding (and thus dramatic) but harmonically rather smooth.

Some tangos (and a few other Latin American songs) like to change mode abruptly. This is more a color change but can be effective between verse and chorus.

1

In composed music just about any jump is possible. If you want it to sound linked, pivot notes are useful. C to E is effective, using E as a common note. Despite the lack of pivot note, C to Bb works very well too. Another trick is to play an extended scale, one key running into another.

If you want contrast and surprise, there are no restrictions! Jump from C straight into F#, B ...anywhere!

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