Nashville Number System
The Nashville Number System is a method of transcribing music by denoting the scale degree on which a chord is built. It was developed by Neal Matthews in the late 1950s as a simplified system for the Jordanaires to use in the studio and further developed by Charlie McCoy. It resembles the Roman numeral and figured bass systems traditionally used to transcribe a chord progression since as early as the 1700s. The Nashville Number System was compiled and published in a book by Chas. Williams in 1988.
The Nashville Number System is a trick that musicians use to figure out chord progressions on the fly. It is an easy tool to use if you understand how music works. It has been around for about four hundred years but sometime during the past fifty years [approximately 1953-2003] Nashville got the credit.
The Nashville numbering system provided us the shorthand that we needed so that we could depend on our ears rather than a written arrangement. It took far less time to jot the chords, and once you had the chart written, it applied to any key. The beauty of the system is that we don't have to read. We don't get locked into an arrangement that we may feel is not as good as one we can improvise.— The Jordanaires' Neal Matthews Jr.
The Nashville Number System can be used by someone with only a rudimentary background in music theory. Improvisation structures can be explained using numbers and chord changes can be communicated mid-song by holding up the corresponding number of fingers. The system is flexible, and can be embellished to include more information (such as chord color or to denote a bass note in an inverted chord). The system makes it easy for bandleaders, record producer or lead vocalist to change the key of songs when recording in the studio or playing live, since the new key just has to be stated before the song is started. The rhythm section members can then use their knowledge of harmony to perform the song in a new key.