If you are going to be a professional player on just about any level, it is absolutely imperative that you have at least a basic understanding of the Nashville Number System. This is a system of writing chord charts and changes created by the studio musicians of Nashville in the 1950's (though some Jazz players would argue that they were using this system prior to that date). In this system each chord is given a number (expressed by a Roman numeral) that corresponds to its scale position. For example, in the key of C major, the first chord is C, and is represented as I. Each of the chords follow in sequence; i.e. D is II, E is III, and so forth. The idea was to make it easier to transpose songs into different keys on the fly, but another interesting by product is that once you get used to hearing how the different chords relate to one another mathematically, it makes figuring out chord changes, and even anticipating changes on songs you are playing for the first time, a breeze.
The best example is a standard blues progression, often referred to as a I-IV-V. For example, if you were playing a blues in C, your I is a C, your IV is a F, and your V is a G. When you listen to enough blues music, you can almost anticipate where the IV will be, or the V, etc. even if you have never heard the song before because you are used to hearing that combination of chords in sequence in the genre. Well, it works the same in any genre once you recognize what that relationship sounds like. In fact, the I-IV-V is the most common progression known to man. Steve Miller's THE JOKER, The Ramone's SEDATED, Johnny Cash's FOLSOM PRISON BLUES, Billy Idol's DANCING WITH MYSELF, and Lynyrd Skynyrd/JJ Cale's CALL ME THE BREEZE are ALL examples of the I-IV-V relationship in action.
To further realize the true potential of this system, and to make sure that you apply it properly, it is important to memorize, and understand, the following chart which will show you which chords are major, and which are minor:
I IIm IIIm IV V VIm VIIdim
While many songwriters take the liberty of putting major chords where minors should be, etc. the hard and fast rules of the chart above will always keep you in key, which is very beneficial to beginning songwriters, as well as accompaniests. Start with the I, and go anywhere you want within the chart from there, and you will always be in key!! Here are a few examples of what the proper chords would be for a few different keys:
Key of C major
C Dm Em F G Am Bdim
Key of G major
G Am Bm C D Em F#dim
Key of B major
B C#m D#m E F# G#m A#dim
Obviously you will have to already have what notes are in what key memorized in order to apply this chart properly....or do you? If you know the first position of the major scale (doe ray me...), then you can use that as your guide; just plug in the chords with the corresponding root notes. You don't necessarily have to think to yourself "In the key of A, the IIIm is the C#", you can just move up to the third note of the scale, and play that note's minor chord. Okay, admittedly, I may breezing through this particular idea too quickly, but if you have any specific questions, please e-mail me.
There is a little bit of contention amongst some players as to how to treat minor keys. Some people just say to go off of the Chords from the major keys (see the above chart), while others give it its own chart which rearranges the numbers, and thus the major/minor placement. I have seen it done both ways, and have done it both ways. For the sake of argument, here is the chart if you give the minor key it's own chart.
Im IIdim III IVm Vm VI VII
I hope this introduction has given you a basic understanding of this vital, yet sometimes confusing, topic. I guarantee you that this is a skill that you WILL use in almost every professional situation you play in. As always, e-mail me if you have any questions.