Finding the Key
Once you know all these structures you have a powerful tool to finding the true key ( or keys ) of a piece of music and therefore which scale(s) to play to improvise over the chords.
Songwriting and Composition
Some musicians use these structures without knowing them at all — they just find that certain chords sound good together.
This is fine but if you consciously use the structures to write you may find you can get ideas which you otherwise wouldn't have.
Remember that you can have more than one structure in a song — indeed this is a good way to get " light and shade " into a piece.
A commonly used rock device is to have a first section in a straight Major structure then a second section using the Pentatonic Rock structure.
This can make the second section sound " heavier " than if you were to launch into it straight away.
Another " light and shade " device is to have a Major section and a Minor section in the same root key [ C major followed by C minor for example ]
Or you could have a minor section followed by a major section using the relative major key
[ A minor followed by C major — ' Stairway to Heaven ' is an example of this ]
A different example of this Minor to Major (and back again ) device can be found in the Lou Reed track' Perfect Day ' — the verse of this song starts on Bb minor then goes through a ' key changing style ' cycle of fifths sequence.
The chorus then comes in with a straight Bb Major key structure — this gives a strong contrast in sound between the verse and chorus.
Once you start analyzing tunes with reference to the various structures you may find that it's much easier to remember how the chord sequence goes — so , for example , I can recall how to play ' Perfect Day ' by simply remembering that it starts on Bbm — does the cycle of fifths routine then changes to Bb Major for the chorus.
I don't necessarily have to memorize the whole chord sequence — just the structure seems to be enough to bring the tune back even though I may not have played it for a long time.
Working out your favourite Tunes Many seemingly different songs actually use the same structural devices — this can make working out chord sequences for yourself much easier.
I can often predict what the next chords are going to be after hearing the first few bars of a song that's completely new to me simply because the first few chords fit into a known structure and so they are already , in a sense , familiar to me.
Analyze some of your favourite tunes with reference to the key structures.