Somebody sent me an email, asking a musical question. I'm not really the guy to be offering such answers, but since it was a fairly simple query, I'll take a shot at it.
The question was about my use of the term "Nashville Notation."
The non-musical among you may skip this one. Those of you who do know music theory, please tell me if I veer off the track.
A bit of set-up:
In western music, letters are sometimes used to stand for notes, chords, and keys. There are but seven whole notes, using A-G to represent them. (There are also sharps (#) and flats (♭) for these notes, and to complicate things, two terms can refer to the same note: A# is the same as B♭. We won't go there.)
Chords are made by hitting more than one note at a time, and in major chords, usually are comprised of at least three notes. The first, the third, and the perfect fifth. (This is determined by taking the letter of the first note, going to the third note in the sequence, then the fifth.)
There also myriad other chords -- minors, sevenths, ninths, thirteenths, suspended, augmented, etc. Whole books of these things are out there. If you know music theory, you can figure them on your own, but a reference book is a good cheat.
Keys go to the note or notes around which a musical piece is centered, thus Canon in D speaks to the key in which the thing was originally written and played. Generally the key of C is the easiest to fool with on a guitar or a piano, because there are no sharps, nor flats in the scale for the key.
Standard musical notation puts these things down on the sheet music and if you can read them, you will have the piece at your beck -- all you need to know about the rhythm, which notes or chords to play, the duration and timing and like that, is right there. It is an entire language.
Not all musicians read standard notation. (Old joke: You know how to stop a classical musician from playing? Take away his sheet music. You know how to stop a rock musician from playing? Put sheet music in front of him.)
The ability to play an instrument and the ability to read music are two entirely different skills.
There are many musicians who can read well. There are others who can't read music at all, but who get by. None of the Beatles, for instance, whose entire ouevre has recently been remastered and re-released, and has blown away all competition to sit at the top of the Hit Parade forty years on, could read music. Amazing.
Ray Charles, who was blind, could read music just fine.
Other forms of musical notation have been devised. Two common ones are TAB (tablature), in which diagrams of a stringed instrument's fretboard show you where to finger the string. Another is Nashville Notation, designed for rhythm players, which uses numbers to represent chord relationships. With this, if you know how to make chords, you can easily shift keys, since the relative positions of chords, especially the major ones, tend to stay the same from key-to-key.
An example: If you strum upon your guitar in sequence the chords G, Em (E-minor) C, and D, you will be able to play about half the rock and roll songs ever written, in the key of G. Most of the blues, you can do with one less chord.
Many rock songs use such chords, in this order, same cadence. All you have to do is change the words. ("Monster Mash," and "Born to Run?" same chords, same order ...)
In Nashville Notation, you don't speak to the key per se, but to the relationship of the chords in it. Thus in NN, the sequence above would be written: I, vi, IV, V. So, if you start with G as I, the Em, counting up from the G, is the sixth chord, written in lower case to indicate minor status. The IV chord is the C, again counting from the G (as I). The V is the D.
Put the number I over a letter, and here is what it looks like:
-----------------I II III IV V VI VII
With Nashville Notation, wherever you start with the I is usually the key. If you know to play the major chords, all you need to do is shift up or down on the fretboard, so, if instead of playing a G to start, you begin with a C, thus:
-------I II III IV V VI VII
Then a I, vi, IV, V chord progression would be C, Am, F, G. It will sound very similar to the first progression, but higher or lower, depending on where you play it on the neck.
Session musicians need to know such things to play with others, and for singers who have a preferred vocal range. Somebody comfortable hitting the notes in D might not have the chops to hit the high notes in B♭.
There you have it. This pretty much exhausts everything I know about music ...