# Use cases for modulating keys?

So I'm just learning how to modulate keys and first of all I want to make sure I understand the concept correctly.... the way I've learned it is basically using a chord function in one scale and turning it into the function in another

So for example, if we're using a I-ii-V-i progression…. Once we land on the i chord, we find what scale uses the notes in the i chord as a ii chord and then play the same functions in the progression in the new scale. So if we were starting in C we would be modulating into A# and even keep modulating if we wanted.... First of all am I understanding this correctly???

If so, at this point it's just kind of a cool trick for me to play with, but I want to find ways to apply it better to music that I make. Right now, the only use case I can really think of is if I wanted to switch to a minor key in order to change the tone of a song, I could find a way to do this where the key we modulate to is minor.

What are some approaches you guys use when modulating keys?

• For reference I make music digitally...

You seem a little mixed up about the relationships diatonic chords have in major compared to minor keys.

Your example hints at this. I - ii - V - i. The key has moved (in a good way - more later) into the parallel minor. So far so good. That's where it starts to fall apart.

I > ii, let's say in key C, is C > Dm. i > ii in key Cm is Cm > Do. It's not necessarily a direct swap. And the same will occur for most of the rest of the chords, too. True, I is equivalent to i, and IV is equivalent to iv. V can, and probably will, stay as V.

Your 'V' of a new key works really well. It just happens that in C and Cm, that V is G. It also works with other modulations. In key C, modulating to key D, use A (V of D), or use G♯ to get to C*sharp;. Or E (V of Am) to get to Am. Major to major is straightforward, and happens lots. Major to minor won't work easily unless there are only three chord involved - tonic, sub-dominant and dominant.

1. Perfect cadence: (I IV V I or iv iv V i)

Starting from any key you can choose a chord of any degree and continue from there with a perfect cadence.

I IV V I VI => i vi V i

1. major -> minor -> dim7

You can also go across the parallel minor chord of a major chord and making a vii dim7 of the new tonic:

C -> cm -> cdim7 -> Db

Try this modulation with arpeggios the 12 chromatic keys.

1. direct modulation:

Sounds always good when passing to the mediant chord.

(Without a cadence you can join directly a song played in C with a transposition or another song in the mediant key Ab or Eb (introducing by the dominant7 also other mediants are common: C , Bb7 Eb etc.)

1. a very common modulation is the key change by introducing the new tonic by the secondary dim7 the 2nd inversion of its vii7/I46 - V7 - I

You seem to be describing two difference scenarios, but I can try to explain both and then it may become clearer to you.

I-ii-V-i... Once we land on the i chord, we find what scale uses the notes in the i chord as a ii chord and then play the same functions in the progression in the new scale.

First, it's better to refer to a key or mode rather than a scale. Also, you really should indicate key or mode when giving Roman numerals, especially when asking about modulation.

So, assuming we are talking about major/minor keys let's start on `C`. `C: I ii V i` shows the tonic chord changes from major `I` (`C` major) to minor `i` (`C` minor.) That's a change in mode rather than key.

The simplest way to play the same chord functions in the minor mode are: `Cm: i iio V i`. In addition to the change of the tonic chord to minor, the `ii` chord becomes diminished `iio` and the `V` chord stays the same (a major triad.)

It might help to name the chords, like `C Dm D C` changes to `Cm Ddim G Cm`.

The second scenario is changing from `C` to `A#`. I assume you mean both keys are major. It's much more sensible to call the second key `Bb` (`A#` major or minor are odd key signatures, you normally use `Bb` major or `Bb` minor.)

So, that's `C: I ii V I` to `Bb: I ii V I` with chord names `C Dm G C` to `Bb Cm F Bb`.

How do you use these ideas?

For the first scenario, the change of mode, I think you understand. A common use is to darken the mood. You could repeat a phrase and switch from major to minor for the second iteration. Or you could start up a totally new section in the minor. You can play with the endings a bit to emphasize the surprise of the change. Instead of `I ii V I` try `I ii V V` hold the `V` then make the mode change `i iio V i`.

The second scenario, the modulation, presents a problem using your example `I ii V I` progression. I may have misunderstood you, but I think you mean to repeat the progression transposed to another key. That process is more like a harmonic sequence. It's tricky to describe the distinction briefly, because there are similarities between the two, and a harmonic sequence can be used as the means to modulate.

A harmonic sequence is when a chord progression is repeated but transposed up or down. It's common to use only a two chord progression in sequence. Sequences can be diatonic `C: [ I6 IV ][ viio6 iii ]` or chromatic `C: [ I6 IV ][ V6/V V ]`. I used brackets to show the sequenced (transposed) pairs of chords. I think you can see why just transposing a progression to another key is like a harmonic sequence.

So, transposing `C: I ii V I` to `Bb: I ii V I` is like a sequence. But, you might ask if a modulation is a change of key, and that transposition goes to a new key, why isn't it a modulation?

I suppose you could call it some kind of direct modulation, but it's more common to have some common chord between the two keys. More importantly the `I` chord doesn't normally start the modulating phrase. Think of it like modulating to a new tonic.

For an example let's start in `C` and modulate to `Bb`. A common chord between the two keys is `F` major. We will have two phrases. The second one will end on a `Bb` chord, but not start with it. `| C: I ii V I | Bb: V6/V V6/IV I64 V7 I |` with names `| C Dm G C | C/E F Bb/D Eb Bb/F F7 Bb |`

It's easier with notation.

The first 2 bar of the modulating sequence could be interpreted in `C` or `Bb`. That's typical of the common chord area of a modulation.

Modulating down a whole step from major isn't really common in classical style, but I wanted to keep with your example.

Jazz does something like `A: ii V I I | G: ii V I I | F: ii V I I` (`Bm7 E7 Amaj7 | Am7 D7 Gmaj7 | Gm7 C7 Fmaj7...`) but it's just another application of the modulating by sequence principle: don't start with `I`, repeat the pattern sequentially, end with a cadence in the new key.