One very strange thing about music is the emotional quality of a plagal cadence. For some reason, repeating the transition from the subdominant to the tonic (or the other way around of course) in a major key, for some inextricable reason the effect is more melancholy than any other progression I can think of (although you could make a case for the old I-iii-IV-iv, as used by Radiohead on 'Creep'). And if you add major sevenths to the chords, well... you might as well just start crying right away.
So this post is just a wee tribute to that strange phenomenon. Here are four seminal examples:
Lou Reed - Coney Island Baby (I-IV-I-IV)
I think that Lou Reed owes a lot of his success to his repeated and skillful use of the old I-IV-I-IV. For example, all three of the slow songs on The Velvet Underground & Nico are based upon it, and a few of the fast ones are too. He often uses it to signify that particular feeling of serene defeat that his characters tend to express, and this song might be the best example.
Erik Satie - Gymnopedie No.1 (IV-I-IV-I)
Only one of the most recognisable melodies in the entire world, you can here really appreciate the power of the major sevenths when added to the voicings, giving this seminal use of the progression that particular dreamy sadness that everyone and their granny loves.
John Coltrane - Naima (I-IV-I-IV)
The coda from this unbelievably brilliant composition is a simple rising melody over the old I-IV-I-IV in, again, major seventh chords (from around 6:30). I used to have a really really big thing for 'Trane, almost a decade ago, although he hardly ever gets listened to now, for whatever reasons. In these late performances it's incredible how he takes such a pretty ballad and slowly destroys it, before reigning it all back in; this process of extension and contraction I always preferred to those pieces which started 'out' and then just stayed at one level of intensity. I used to have a particular Coltrane recording, Live at the Village Vanguard Again! and on this composition it featured one of the most ridiculously mental solos from a young Pharoah Sanders, sounding at times like a baby being fed through a blender feet first. I loved that record. I loaned it to someone, it never came back, and it seems now that the record label have discontinued it.
Richard Wagner - 'Liebestod' from Tristan & Isolde (IV-I-IV-I)
And; if you want to know what it feels like to be roughly slapped about the face by a plagal cadence, then Wagner's your man (the big one comes at 5:00)
A couple of things to note; the scene here is of Isolde literally dying of grief, a fatal romantic swoon, the melancholy-major tonality perfectly suited to evoking the required mixture of passion and abjection. In this case the sixths of the chords are prominent rather than the jazzier sevenths, resolving downwards as an outrushing torrent of vitality (or Will, of course). This particular plagal cadence comes at the end of an entire opera's worth of teasing, deferred non-cadences; you need only look at Simon Rattle's face to get the idea of just how much of a release it is for that tonic chord to finally 'arrive', as it were... dirty boy.