Chord Numbers - Learn Guitar Theory Part 6
Chord progressions are often referenced in sequences of numbers. Roman numerals to be precise. If you've ever been to a music jam and heard musicians saying something like "let's do a one four five in G," the other musicians will know exactly what to play. The numbering system itself is very straightforward.
In the lesson "what key is it in" we took a look at how chords are constructed from notes in a scale and how the resulting chords follow a certain order. For instance, the basic three note chords (triads) go in this order.
Major - minor - minor - major - major - minor - diminished
These chords are numbered from one to seven, starting from the root using Roman numerals. Upper case is always used for major and lower case for minor. Doing this allows us to communicate what a chord's function is within a particular key. In the key of C the chords and their associated numbers will look like this.
Cmaj - Dmin - Emin - Fmaj - Gmaj - Amin - Bdim I ii iii IV IV vi vii
Armed with this information, if someone says play a one four five in C (I IV V), I'll know it means the chords C F and G. Or a one three four five (I iii IV V) I know it means the chords C Em F G, etc.
There are many reasons why this can be very useful but as a beginner, the reasons might not be so apparent. As you become more experienced and more knowledgeable with music theory you start to see the importance of using numbers to describe a chord's relation to its key.
The chords position is relative to the key. So if for example the key is A minor (the relative of C major) a I-iv-v in A minor will be Am-Dm-Em. So even though A minor shares the same chords as C major, the chords won't be the same. This makes more sense when you see them together...
Key of C major
Cmaj - Dmin - Emin - Fmaj - Gmaj - Amin - Bdim I ii iii IV IV vi vii
Key of A minor
Amin - Bdim - Cmaj - Dmin - Emin - Fmaj - Gmaj i ii III iv v VI VII
Using chord numbers also makes it easier to transpose a key. When you know the chord progression in it's numbered order, it makes it easier to translate to another key. As long as you know the scale notes, you just keep the same order and apply it to a different scale. For example, we know the I-IV-V in C is C F and G. If we want to change key to F major then we can count the notes in the scale.
F major scale = F G A Bb C D E
so a I-IV-V in F is F Bb and C.
There are many more reasons to learn chord numbering but they involve more advanced theory. If you want to learn more about music theory and see how this stuff can be so useful then I would recommend two Idiots Guides that in my opinion do the best job of making this stuff easier to understand. The two books go hand in hand and I would recommend getting both of them. You can get them directly from Amazon using the links below.
Learn Music Theory The Easy Way
If you want to get a better understanding of music theory then I highly recommend these two books. I have found them to be among the easiest to follow but without lacking any information.
The Idiots guide to music composition compliments the theory book perfectly. It answers all the questions you expect to see in other theory books but never do. In fact, if you already have a reasonable understanding of music theory then you should check out the composition guide. Learning theory is one thing, how to use it is another. The Composition Guide plugs the gap between theory and it's actual use in music.
The Complete Idiots  
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The Complete Idiots  
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