In reading some other posts, and in discussing Forms in the previous articles, I realized something that I might be forgetting to mention that many might find helpful.

In Common Practice Period (CPP) works in the Tonal style - and music that continues to be written in a Tonal style - we look at a very important formal and structural characteristic - Tonality!

That's why all these theorists call Tonal music Tonal - it's about Tonality - Key centers! So many students get bound up in chords and notes and progressions and part writing and analysis that they get a misleading impression of what "tonal" music is. They think tonal music is music that uses chords, and chord progressions, and scales, etc. But what Tonality is *about* (and what makes it different in a huge way from earlier Modality) is Key Centers.

Most tonal music uses Key Centers as important guideposts, both in large and small scale manners.

There are virtually no (i.e. very few by comparison) CPP works in one key. Virtually all of them have at least two keys.

Many people know that the two CHORDS of Tonic and Dominant are structural guideposts, but, they should also be aware that the most important KEY besides the Tonic (or Home) key is the key of the Dominant.

That said, most Tonal music uses Closely-Related Keys.

Finding the closely-related keys to your Home key is easy. They are all Six chords in a Major key besides the diminished one (viio) and all Six cchords in a minor key (natural minor) besides the diminished one (iio)!

Another common way to describe closely related keys is that they differ in key signature by only 1 accidental. So for example, F major has 1 flat, so the next two closest keys are one with 0 flats, and one with 2 flats, making it C Major and Bb Major.

Yet another way to look at this is, the closely related keys are I IV and V and their relative minors for Major keys, and i, iv and v and their relative Majors for minor keys. Thus:

V - iii
I - vi
IV - ii

and

v - VII
i - III
iv - VI

(notice that the relative keys in each pair also are I, IV and V or i, iv and v!).

Note that in minor, it's the key of v, NOT V that is modulated to. Thus, in cm, gm is closely related, not G Major (besides, GM is 1 sharp, and cm is 3 flats - so that ignores the methodology above).

If a Major key piece is going to modulate (and it will in tonal music!), it is MOST LIKELY to modulate to V, and then second most likely to modulate to vi.

V
I - [vi]

In minor key works, III is the most common key to which to modulate, with v being next most common.

[v]
i - III

Some forms, such as Marches, frequently move to the key area of IV/iv in certain sections, but overall, it's no more common than other possibilities. EXCEPT, many pieces will often focus on the key area of IV/iv at the end. In a sense, this is similar to an extended Plagal cadence idea.

Now, Tonal composers were about Key Centers - this means that they viewed the KEY CENTER of "C" as the "KEY". Today, we have a tendency to see C Major and C minor as different keys (largely because we're taught key signatures mean the key from an early age) but composers of the past thought more like "Key of C, Major mode" or "Key of C, minor mode". This mean that, to them, changing from C Major to c minor is not a modulation, but a "change of mode".

This opens up a whole second tier of closely related keys. Taking C as example:

Major
G - em
C - am
F - dm

minor

gm - Bb
cm - Eb
fm - Ab

If we "connect them" we get:

Bb - gm | G - em
Eb - cm - C - am
Ab - fm | F - dm

Now, this does not mean Eb and am are closely related. They are not. But, they are more closely related through the key center of C than C Major and F# Major are.

But what it does mean is, you are likely to see a C Major piece change mode to C minor, then modulate to Eb Major, move to gm, change mode to G Major, and then modulate back to C.

Did you get all that?!?

Now there are also distantly-related keys. In general, composers didn't modulate to distant keys unless they either did so through a succession of related keys, or for special effect.

There is one set of more-distantly-related keys though that are common, especially as we move further into the Romantic period.

These key relationships are called "Chromatic Mediants" (as are any two chords that share this relationship). Mediant chords are III/iii (and submediant are VI/vi). Chormatic means there's a chormatic note (non-diatonic note) in the new key.

The easy definition is that Chormatic Mediants have Tonics a m3 or M3 apart, and are the same quality. Therefore:
C - E
C - Eb
C - A
C - Ab

Are all CM related keys (those are all major above). For cm:
cm - am
cm - Abm
cm - em
cm - Ebm

Hopefully you can see that for example, C and Ab are linked though cm - C changes mode to cm, then Ab is in cm's closely-related family. But the deal with Chromatic Mediants is they typically "skip" the chords that would make them more closely related, and just go directly to the new key.

So "close", but "not as close" as the mode-change families above, and not anywhere near as close as the closely-related family at the top.

Anything else is Distantly related.
C - F#
C - F#m
C - Dbm

etc.

Now, this is not to say composers didn't use distantly related key relationships - they did. But in general, and by far most commonly, composers used closely related keys to modulate to.

I see many pieces posted where a composer has a section in C, then one in E major, then one in Bb minor, and they make no sense and sound "stuck together". Also, I see things where each section is in C major (where they likely transposed their stuck together parts). Neither of these are common historically, especially in tonal music (yes, modern pop music has weird key changes (if any), and very often has only one key throughout, but this is an exception historically).

So in the previous discussions of form, I hope I made it clear that common modulatory schemes should be learned, and then deviations can be explored, but it's best if those deviations occur in increments of modulating to less common closely-related keys, then not-so-closely related keys, then when you're ready, distantly relatec keys.

Hopefully this sheds a little light on this oft-neglected topic.

fine.