Diatonic Theory - Uses For Roman Numerals

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The Roman Numerals are used the write out progressions that are Key Related, but not related to any key in particular. IOW...say you've written a song are going to present to a singer...but you have no idea what 
the singers range is, or if the key you've written it in will be the best key to get the singer sounding their best. And, let's say you also need to have basic chord charts for everyone in the band to follow along.

Let's say this is our chord progression and you wrote it in the key of C Major:

||: C F | Am Em | Dm G :||

Ok, now you play it together and it's to low or to high for the singer. Well, you are going to need to 
change the key to fit the voice. So, now EVERYONE has to rewrite their chart to reflect a new key, and it 
may take trying a few keys to find one the singer is good with.

All this rewriting takes time, and depending on how long the chart is...it could take a long time with all 
the rewriting, double checking, and doing it all over again if needed.

Here's a simpler (and common) way people deal with this:

They write out the original chord chart in Roman Numerals and fit those, the Root notes of the chords, to 
the Key that is decided on.

So, if you look at the chord progressions we had (in the key of C Major):

||: C F | Am Em | Dm G :||

You can write it in Roman Numerals like this:

||: I IV | vi iii | ii V :||

Hopefully that makes sense when looking at the Roman Numerals I listed in Part 5. So, if you changed the 
key to G Major (G A B C D E F# G) you just plug in the correct/new Roots and play them as Major or Minor depending on the Upper and Lower lettering and you've changed the key on the fly with nobody needing to rewrite anything. So, in the Key of G your progression would be:

||: G C | Em Bm | Am D :||

These are VERY common practices that you'll encounter as you delve into Keys, Modes, etc...and they are all part of what's called "Diatonic Theory". Look that term up and you'll start running into these things and more.

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