It's a shorthand chord charting system based on the principals of diatonic music theory, specifically identified by it's use of Arabic Numerals to denote diatonic harmony (instead of Roman like everyone else since roughly 1800A.D.); but it's only a chord charting system for easy transposition, not a replacement or the source of Numerical Analysis in music.
Numerical analysis has been around for centuries... multiple styles of music have some form of this specialized for the style... most predate it actually, I'm not aware of any others being declared a "system" and marketed as a replacement for basic music theory; some sources I've found on it treat it as such (basic theory)... here are some site links covering this "new math"... If you read through the comment sections on the different pages you'll see that this is tends to create those "religious" type arguments.
Call the IV the 4 chord or the "four" chord or hold up four fingers... or heck... I could write out a chart using |||| for the four chord (like 1=I=|, 2=II=||, 3=III=|||, 4=IV=||||), I can't draw a line across the 4 pipes to represent 5 (or V). It's been in use for centuries.
Why do I say this causes "Religious" arguments? Because even while every one of these sources identifies it as a system for charting and transposing... There is a large group of musicians who apparently believe that this is the source of the concept of referring to chords by their number or diatonic function. Obviously everyone else in the world familiar with Music Theory knows this is wrong; it's like believing the Sun is the center of the Universe. This is a quick easy way to use basic theory (written slightly non- traditionally) used to notate songs in an easy to transpose way. If you've never run across numbering chords before it didn't originate in Nashville; this particular "style" of it did, but the concept of numbering chords in did not.
Referring to the "IV" chord as the "IV" chord isn't new just because you use "4" instead of "IV"... People of every style of music based on the 12-note western system call the IV chord the IV chord (or "4"... "tomato=tomato", "a rose by any other name is still a rose").
Those caveats aside it's a commonly used method for country/western music... if your belief system encompasses that as the entirety of music, you might believe this is actually music theory instead of a Notation system... If you feel that way... this IS theory. ("We have both kinds of music here, Country AND Western" - The Blues Brothers). It really only works well for simple songs that only use diatonic triads; you can do a whole lot with that, but it essentially ignores anything more complex ("nothing to see above the 5th of the chord here... see... there's nothing up my sleeves"), if you like to alter your 7th, 9th, 11th, or 13th chords, they don't exist in this particular "Religion" that's not "Music" apparently (I'm being silly to make a point... don't try to Notate Coltrane's "Giant Steps" changes or Metallica's "Master of Puppets" using this, while both of them are wildly different from each other, neither style can be easily Notated using the Nashville Numbering System, it's not made for that, however... I can analyze both using Roman Numeral Analysis... because it actually IS a system of Musical Theory Analysis).
As you read through all these explanations keep in mind that it's pretty much limited to diatonic Triads... Oh, he's got a vii7b5 (7- halfdiminished 7th if you're not familiar with Roman Numerals), that's a diatonic 7th chord, but every other thing deals with altering just the triad. It introduces slash/compound chords for a little more complexity... a 5/4 would be a IV7suss9 sound (5-/4 would give us a b9 as the minor 3rd in the V chord 1, m3, 5 of the "5-" chord over the root of the IV chord finally! some chromatic alteration!).
There's an interview with Chas Williams (he has a book out on it too) at GuitarThinker.com
I play guitar, slide guitars and dobro. After a couple of semesters at Berklee College of Music, in 1979, I moved to Nashville. I learned the Nashville Number System while working in Steve Bivin's writer's nights bands and doing demo sessions. At a writer's night, the band backed up 20 to 30 songwriters. We would play their original songs with no rehearsal, just number charts. We wrote most of the charts ourselves and recieved great ear training taking dictation from thousands of song demos.
Check out that interview...
Nate: What is one thing about the Nashville Number System you think most people that are new to it don’t understand?
Chas: A lot of people assuming that the 2, 3 and 6 chords are minor; like in a diatonic chord progression. However, in the NNS, no chord is minor unless you designate minor with a minus sign after the number.
That's interesting... I wonder if this grows out of how a pedal steel guitar works, I actually studied with a pedal steel player (2nd guitar teacher), at the time we didn't get up to that point. My point with that is that you'll notice he's in the picture with a dobro.
It is a system for short hand charting in a very informal manner, It came about as a quick way of outlining country/western/rock & roll songs in the 50's. It was developed by Neal Mathews, Jr.
As a member of The Jordanaires, he worked with artists such as Patsy Cline, Red Foley, Johnny Horton, Ferlin Husky, Jim Reeves and George Jones. They group also served as backup vocalists for pop music artists such as Steve Lawrence and Eydie Gorme, Connie Francis and Julie Andrews. They are best known, however, as the backup vocalists for Elvis Presley for 15 years.
Matthews and the group also toured extensively around the world and recorded a number of their own albums, winning a Grammy Award for Best Southern, Country, or Bluegrass Album.
At this point a couple things should be pointed out, depending on the style of music you like and are learning, this may or may not work well for you. If you're a slide player you'll love this, or if you are playing guitar where you are changing keys with a Capo.
This is a method of Notation that is really useful for transcribing songs of these styles and representing them in a pretty informal shorthand, as noted in the Wiki article, this is similar to Figured Bass.
Figured bass, or thoroughbass, is a kind of integer musical notation used to indicate intervals, chords, and non-chord tones, in relation to a bass note. Figured bass is closely associated with basso continuo, an accompaniment used in almost all genres of music in the Baroque period, though rarely in modern music.
Other systems for denoting or representing chords include: plain staff notation, used in classical music, Roman numerals, commonly used in harmonic analysis, macro symbols, sometimes used in modern musicology, and various names and symbols used in jazz and popular music.
If you wanted to rock out some Baroqueimprovised keyboard accompaniment you should probably check that out.
If you're a Jazz guy, you'll probably want to learn Roman Numeral Analysis,.
In music, Roman numeral analysis involves the use of Roman numerals to represent chords. In the most common day to day use, Roman numerals are used so the musician may understand the progression of chords in a piece. The actual chord names (e.g. C, F, F♯, etc.) are substituted in place of the Roman numerals once the key of the piece has been decided. This allows progressions to be easily transposed to any key. In essence, it is a way to abstract the chord structure of a piece.
Conceptually they are all explanations of the same thing; the Nashville Numbering system and Figured bass are both styles of Musical Notation.
Roman Numeral Analysis is analysis though, it's not notation; something completely different conceptually... however it's so related that all of these things fall under Music Theory.
In the end... it's a fundamental system of shorthand notation for musical styles originating out of the Nashville tradition. The only criticism I have of some of the explanations of this probably stem from the marketing strategies that have built out of it. The fundamental theory behind the music is the same, this isn't the source of numerical analysis of music in any way, it's a charting style that is based on historical numeral analysis and is specialized for a particular style of music. If you are really searching for a deeper fundamental understanding of music, the theoretical harmonic possibilities that are available... then it's a little limited, that's not it's purpose (to explain theory), it's just a set of directions. (don't let my use of the word "just" disturb you; directions are really important).
If you're attracted to this idea because of the analytic/theoretical ideas it may have introduced you to and not as an attempt to explore the "Nashville Sound", you may be really interested in music theory and the historical Roman Numeral style of analysis.
If you just like learning different styles of music, and tend to study them all, this is how the Nashville sound works. All styles of music tend to have some kind of shorthand charting style, particularly for idiomatic instrumental oddities that vary from style to style, region to region.
In music, the concept of idiom has been applied to a wide variety of phenomena; however, the
term is commonly associated with the use of distinctive instrumental resources. The mechanics of musical
instruments commonly influence how the music itself is organized. Like spoken utterances, musical
passages can be characterized as more or less idiomatic depending on the extent to which the music relies
on instrument-specific effects.
Ethnomusicologists frequently point to instrumental idioms as having had marked impacts on the
character of music-making in different cultures (e.g., Baily, 1985; DeWitt, 1998; Yu, 1977; Yung, 1980).
Similarly, historical musicologists have often pointed to physical characteristics of performance, especially
in relation to arrangements of musical works made for different instrumentation (e.g., Dreyfus, 1998; Le
Guin, 1997; Mohr, 1972; Morehen, 1994; Parkins, 1983; Shepherd, 1995; Shao, 1997; Ung, 1981). Jazz
and blues musicians often stress the importance of idiomatic instrumental techniques in improvisation (e.g.,
Richardson, 1996; Sudnow, 1978, 1979; Walser, 1993). Electroacoustic musicians can identify the vintage
of electronic works by the characteristic sounds or gestures associated with specific hardware devices or
formerly popular algorithms (e.g., Henry, 1970). Musical genres have been identified in which the
idiomatic aspects of one sound source are imitated by another – such as the vocal imitation of the Greek
gaida bagpipe by traditional singers (Sarris & Tzevelekos, 2008). Musical idiomaticism has been addressed
in a variety of descriptive and statistical ways by music scholars (Horton, 1986; Jiranek, 1971).
The point is... every style of music, and every instrument for that matter has it's own "Dialect", (metaphorically speaking) We're all speaking the same language of music (within the context of "Functional Harmony"); all these different charting styles are like idiomatic dialects of functional harmony, they specify all the idiosyncrasies that are introduced by specific instrumentation and styles.
Learn whatever style of notation that is germane to your Genre... but keep in mind the bigger picture of music theory and don't confuse the two, instead realize that they have to correlate and agree or there is something wrong somewhere in your understanding.
I originally came across this in a search for different ways to notate music online, this pretty much circumvents the need for special software for really basic songs as you need no staff notation... for the same reason, it's quite limited (but it's supposed to be).
Why do you see all these people arguing about this and music theory in the comments sections on these pages? Because to those familiar with traditional Music Theory & Roman Numeral Analysis... this looks exactly like poorly done harmonic analysis by someone who's never studied it (sorry, but at first glance that's what I thought it was), it isn't that at all... unless that's the extent of your musical knowledge, in which case... it kind of is... Here's a metaphor using computer programming as an example of a cranky old guy like me arguing over a "technical" religious argument...
("WTH! You can't call over-optimization OOP... OOP means Object Oriented Programming.... it's the kind of programming that you're doing incorrectly because you don't know what it is, because you don't know what it is you've screwed your web page up and now you're calling it "Over Optimization" and want to call that "OOP"... well if you knew OOP in the first place (object oriented programming), we wouldn't be having this discussion of why your web page doesn't show up in Google... it's because you don't fundamentally know what OOP is, and you screwed it up and now Google doesn't understand what you're trying to do... it's not going to help to get a bunch of people together on a message board and make up a new acronym for something that doesn't exist and discuss that instead of actually learning Object Oriented Programming" NOTE* If your SEO guy thinks OOP means over-optimization... fire him, at the least, send him to school, he's missing the forest for the trees...)