: Harmonizing All Music with the I,IV and V Chords? I read somewhere that any melody line can be harmonized using the I,IV and V chords of the major scale. The reasoning behind this being that any possible note played in

I, IV and V contain, between them, all the notes of the scale. So there's a degree of truth in the statement, but only if you restrict the melody to scale notes. And only if you want a possible harmony, not a good one!

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I read somewhere that any melody line can be harmonized using the I,IV
and V chords of the major scale. The reasoning behind this being that
any possible note played in the melody will exist in one of these
chords.

Firstly, that reasoning only makes sense if the imagined melody line that stays within the major scale. The idea will 'fail' for any melody that uses notes outside the major scale.

Even with this restriction then, how profound is it to say that any major scale melody line can be harmonized using the I,IV and V chords?

Even if we restrict ourselves to sets of chords that contain the tonic (I), there are quite a few sets of chords from the major scale that contain all the notes of the major scale:

I,ii,iii

I,ii,V

I,ii,iii,IV

I,ii,vii (dim)

I,IV,V

I,ii,iii,V

I,ii,iii,vi

I,ii,IV,V

I,IV,vii (dim)

I,ii,iii,vii (dim)

I,iii,IV,V

I,ii,IV,vii (dim)

I,ii,V,vi

I,vi,vii (dim)

I,ii,iii,IV,V

I,ii,V,vii (dim)

I,iii,IV,vii (dim)

I,ii,iii,IV,vi

I,ii,vi,vii (dim)

I,IV,V,vi

I,ii,iii,IV,vii (dim)

I,ii,iii,V,vi

I,iii,vi,vii (dim)

I,IV,V,vii (dim)

I,ii,iii,V,vii (dim)

I,ii,IV,V,vi

I,IV,vi,vii (dim)

I,ii,iii,vi,vii (dim)

I,ii,IV,V,vii (dim)

I,iii,IV,V,vi

I,V,vi,vii (dim)

I,ii,IV,vi,vii (dim)

I,iii,IV,V,vii (dim)

I,ii,iii,IV,V,vi

I,ii,V,vi,vii (dim)

I,iii,IV,vi,vii (dim)

I,ii,iii,IV,V,vii (dim)

I,iii,V,vi,vii (dim)

I,ii,iii,IV,vi,vii (dim)

I,IV,V,vi,vii (dim)

I,ii,iii,V,vi,vii (dim)

I,ii,IV,V,vi,vii (dim)

I,iii,IV,V,vi,vii (dim)

I,ii,iii,IV,V,vi,vii (dim)

The set {I, IV, V} is one of the smallest suitable sets of chords, consisting of only three, but there are other sets of three chords that also contain all the notes of the major scale. The question arises, then : why single out {I, IV, V}?

There must be another criterion the author of this proposal is going by, other than simply that the set of chords must contain all the notes in the major scale. In other words, {I, IV, V} is somehow seen as providing more satisfying cadences and harmonic movements than those other groups...

...but then, if providing satisfying harmonic movements is a criteria, why restrict ourselves only to the three chord groups? Why not make a similar statement about {I,IV,V,vi}?

On the other hand, if we restrict ourselves to the three chord groups, it's possible that for some melodies, {I,ii,V} or {I,ii,vii (dim)} would provide more appropriate possibilities.

Possibly the significant thing about I, IV, V is the fact that 1) in itself it is a common set of chords used to construct a progression (due to the strong relationship between I and IV and I and IV), and 2) it happens to be one of the smallest sets of chords that contains all the notes in the major scale.

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The I,IV,V are the basic chords in a scale. The other chords (ii,iii,vi, vii) can be 'created' from these chords by substituting some notes for some other. Let's take the C major scale:

I: C,E,G
IV: F,A,C
V: G,B,D

The remaining chords are:

ii: D,F,A -- Take IV, remove C and add D.
iii: E,G,B -- Take V, remove D and add E.
vi: A,C,E -- Take I, remove G and add A.
vii: B,D,F -- Take V, remove G and add F

So, in a way you could say that you could harmonize a simple song like Mary had a little lamb using only the 3 basic chords. You can use the other 4 chords when you want to spice up things a bit. If you have a song that sounds boring only with the basic chords, you can substitute some of the chords with some other. This is really common in music. An example you might have seen:

There is the usual progression I IV V, which is really really often substituted by I ii V.

Also, it is common after V to see vi instead of I.

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This is generally true of a melody in a major key. However, things can be more complicated in actual usage.

If the melody contains chromatic notes that are not in the key, the basic three chords may not work. Also if the melody modulates into an entirely different key than the original key, it won't work either. There are many songs where the melody does not stay in one key the whole time. In that case you need to identify what new key or keys that the melody has modulated into, and use the chords from the new key to harmonize that part of the melody.

You should also learn about the distinction between homophonic texture (the kind of texture used in your own example, where you have a different chord for every syllable or note in the melody) and melody with chordal accompaniment, which is what people are usually talking about when they say using three chords to harmonize a melody. In the latter, you usually only have one or two chords per measure, and each chord rings out for a few beats while the melody notes change above it.

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I read somewhere that any melody line can be harmonized using the I,IV
and V chords of the major scale. The reasoning behind this being that
any possible note played in the melody will exist in one of these
chords. My question is whether this is true, and if not, in what
context would this idea fail?

I'm going to try and take a stab at this. I'll try and explain the core concepts of this question so you have a better idea of what is going on.

Firstly lets take melody.

Melodies when written correctly are built on chords. This is a big misconception that seems to be widespread but the first thing you have to decide on when writing a melody is the chord progression.

Ambiguity in regards to chords leads to melodies that have no proper foundation to build on. This is not chord in the way we guitarist think of them. When you jump from c to e to g in a melody then it is clear that the melody in that bar is built on a C Major chord.

There really is no good melody that would have only Primary Chord. Your natural chord progressions for melody writing would simply not allow you to only have Primary Chords in them.

It is simply just a sign of a good melody and harmony exercise to include Secondary Chords as well.

Harmony

Now harmony is when you are given a melody and asked to harmonize the melody for four voices. This is not really how music is written. It is all just an exercise to explain the core concepts of how multiple voices interact with each other.

When you think of harmony you need to think about how the chord interacts with the previous and the next chord. You need to think about the spacing, the doubling of the notes, the resolutions and the form.

Trough this all you have to write a melody within the boundaries of the rules of good harmony. You need that melody to make sense and interact with the given passage.

Where melody is just a single line of notes harmony is four lines of notes that all have to come together.

Now you ask...

I read somewhere that any melody line can be harmonized using the I,IV
and V chords of the major scale.

I would say that this is somewhat general. Maybe if a melody only consisted of these three chords then maybe you could harmonize it with only these chords but as I say your melody chord progressions should be more than just Tonic, Sub Dominant and Dominant.

The reasoning behind this being that any possible note played in the
melody will exist in one of these chords. My question is whether this
is true, and if not, in what context would this idea fail?

The choosing of chords for harmonizing is much more than just choosing one of the three you mention. This is a gross over simplification of good harmony.

There is a plethora of chord progression that are important for harmony that do not have I, IV or V in them. You have the passing 6/4 built on the Leading Tone chord which you will never learn to use in this way.

You have a Imperfect Cadence that can be ii - V. You have an Interrupted Cadence that ca be V-vi. You have a lowered Leading Tone progression that is I - III - Iv with vi sometimes being substituted for I.

All of which are things that are often used in harmonization and all of which you will never learn with this misconception.

Truly harmony is much more than just a melody written of I-IV-V

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I think theoretically yes, but it won't sound completely "right". Because say in a major key, if you took I IV V, these would only be the major sounding chords. So in a song that always sounds happy this would work.

But sometimes you want to throw in those minor chords (the ii,iii,vi) in order to create a sense of sadness that you otherwise couldn't with the I IV V.

After the I IV V the most common is the vi and then the ii.

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