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.