In live band situations musicians will communicate chord progressions often referring to numbers. For example, you turn up to a jam night and the band decide to play a song you’ve never played before. The conversation could go like this:
You: “What are the chords?”
Bandmate: “One, Four, Five in the key of G”
When you hear musicians say this, they are referring to the chords as numbers (much like the numbering system we have for finding the notes for different chord types). Only when referring to the chords in a particular key, the assumption is that you already know what the chord type will be. This numbering system is usually written down using Roman numerals. Lets look at the key of G for this example:
Now lets assign numbers to each of the notes using Roman numerals:
I always means major chord
II always means minor chord
III always means minor chord
IV always means major chord
V always means major chord
VI always means minor chord
VII always means diminished
So the chords in the key of G would be:
So when a bandmate says: “One, four, five in the key of G”. You know that he/she is telling you to play Gmaj, Cmaj and Dmaj. As of course the I, IV and V would mean different chords in other keys, it works as a great way to communicate chord progressions in a live situation.
Like the method used for finding notes of a major scale using the T-T-S-T-T-T-S system we can apply this same type of logic for finding the chords types within any key. When the notes of a major scale are written out in sequence order starting from the root note (in the key of C, the root is C), The chord types always follow the order:
This is you popular music palette of chords to write chord progressions (we'll talk more about that later). This is the language of musicians and a very useful concepts to learn.
Hope you have found this useful, let me know in the comments :)
Taken from my book series 'Six String Enigma'