John Mehegan (1916-1984) was a jazz pianist, critic, educator and author of many books on jazz. He came up with this unique style of roman numeral notation that represents a harmonic approach to jazz facilitating transposition in all keys.
This app demonstrates how the Mehegan system works by showing regular chord notation with equivalent roman numerals side by side. Real-time modification of chords can be done in all keys.
A second page allows viewing roman numeral chord charts from the iReal Pro app (a paid app that must be installed separately). In order to import into Mehegan you need to go the iReal Pro and share a chart using the "Open in..." option or by pasting it to the clipboard.
When using the Mehegan App with iReal Pro certain text entries are used for key changes. In the iReal Pro app, these have to be typed in at the beginning of the staff text entry for any chord where a key change takes place. For example, "/1/" tells the chord progression to move up a full step, "3" down a major third, etc. For more information on this as well as numerous examples, including Rousseau notation, check out the examples on the supporting web page and John Mehegans books.