Nashville number system
n.— «On a legal pad he had “charted” the five songs, using what’s called the Nashville number system, a time-saving trick that assigns a number to each chord in the do-re-mi musical scale, making it easier for musicians to change keys without rewriting the chart. We listened to my originals one by one.» —“When Walter Mitty Met Conway Twitty” by Bob Baker New York Times Mar. 25, 2007. (source: Double-Tongued Dictionary)