Ok, in today's strange installment I'm going to look at using the IV and V chords in composition and some possible substitutions.
Let's use C as the root of our one chord and we'll work in major and minor keys.
So, starting with the basic major key I-IV-V we get C F G. This is the basis of Western Harmony so I'm not really giving away any secrets here.
For the minor key we have Cm Fm G.
This chord progression is generally interchangeable with the I-II-V (i.e. C Dm G), often with the II chord in first inversion (C Dm/F G) to maintain the root progression.
One fairly well known substitution for the IV chord is the Neapolitan Sixth. In it's most classical form this would be the first inversion of a major chord built on the b2 of the minor root.
So we might go from Cm Db/F
Of course like many of these little ideas, rules become relaxed and the Neapolitan Sixth can be used with the major I chord and it can even be used in root position.
This gives us Cm Db, C Db/F and C Db. A nice example of the last movement (C -> Db) occurs in John Williams' Raiders March.
Now an interesting point is that if we make this a dominant seventh chord (ie Db7) and keep it in root position then this chord becomes a tritone substitute for the V chord. Allowing C Db/F Db7 (or even C Db Db7) to act as a I IV V.
Another area of chord substitution comes from accessing the parallel major or minor scales.
For instance in C major we might consider using a minor IV chord (i.e. C Fm).
Also, it is possible to borrow the IV and V major chords from the major root of the parallel minor key.
So, we are in C major and wish to borrow the chords from Cm. Cm is the equivalent of Eb which has IV and V chords of Ab and Bb. We can therefore substitute the chords of Ab and Bb as IV and V chords, respectively, in the key of C major.
Another way of looking at that last post is that bVI and bVII can be used as substitutes for IV and V in a major key. Chord progressions like C Ab Bb are very common in film music.
A similar progression might be to take the IV and V chords from the Dorian mode allowing us to use the bIII and IV chords. This would give C Eb F as a progression.
Again, these are all just ideas and hopefully they'll encourage more conversation from you great people here!