Musicians and psychologists hold slightly different practical definitions for what is known as the contrast principle. A good description of a psychologist’s use of the term might be something like the following, which comes from the JackMalcom.com website:
If you put your left hand into a bucket of cold water, and your right into hot water, and then plunge them both into a single bucket of room-temperature water, a funny thing will happen. Your sensations will reverse: your left hand will feel warm and your right hand will feel cold. There’s no magic in it, it’s simply the contrast between the starting and ending points of each that makes the difference.
In music — and in fact most other art forms — the contrast principle, at least in common usage, refers to opposite qualities being in relatively close proximity.
For example, a verse melody might feature melodies made up of downward-moving phrases, contrasting with a chorus melody that features upward-moving ones. A verse might be quiet, and a chorus might be louder. Or a bridge progression might feature mainly minor chords, switching to mainly major for the chorus.
But as a songwriter, you might be well advised to think about the psychologists’ use of the term, especially when you think about how much contrast is needed.
Here’s a quick look at what I mean.
We know that a chorus melody will usually be higher than a verse melody. (Example: “Rolling in the Deep” – Adele) But if a verse melody dwells on lower notes, the chorus does not need to be much higher in order to come across as being higher. In other words, a chorus that’s only a note or two higher might give the impression of being a lot higher than it really is. Moving slightly higher might be all that’s necessary.
A good example of this is Jason Aldean’s “Big Green Tractor“, the verse for which spends considerable time sitting in and around G#. The chorus moves up to dwell on the note B — only a minor 3rd higher. But that minor 3rd interval higher, with a louder and busier instrumental accompaniment, gives the impression of being a lot higher, and so the chorus achieves considerable energy because of it.
A bridge will often dwell on minor chords if the song’s chorus features mainly major chords. But in reality, a bridge often only has to start on a minor chord or two to give the impression that the entire section is minor. Christina Perri’s “A Thousand Years” uses a mainly major verse and chorus. The bridge doesn’t change key at all — staying in Bb major. But the first two chords are Cm and Gm, two chords that come naturally from that same key. But starting the bridge with these two chords makes it sound as though the tonality has switched completely to minor.
We know that a chorus instrumentation tends to often be busier, higher and louder than the verse and/or pre-chorus that precede it, and a good example might be Fun’s “We Are Young“. In fact, there really doesn’t need to be much of a change at all to give the impression that a big change has happened. Foster the People’s “Pumped Up Kicks” shows how just a slight build in production/instrumentation at the chorus is all that’s often needed.
Your Own Songs
For your own songs, the benefit that comes from holding to the “slight-change-is-big-change” technique is that you have a better chance of having your entire song glue together a bit better. When a chorus is higher, but not all that much higher, than the verse, there is a stronger sense of continuity that runs through the entire song.