The Nashville Number System is a musical notation method that uses numbers to represent chord progressions, allowing musicians to play in different keys without rewriting charts. It assigns Arabic numerals (1-7) to chords built on each scale degree of a major key.
Within Chord Sheet Maker Online you do not actually write the numbers, but can transform the resulting chart to show numbers instead of regular letters.
There are three steps required to make a Nashville Number Chart ...
See some more Nashville Number System examples.
In the settings you can choose your preferred display, i.e. whether you prefer Arabic or Roman numerals. And how minor chords should be represented.