KeyJ: Music Theory

KeyJ is a musical translator that speaks your language. Just dial in a key or scale. Musicians see a lit up fretboard or piano, DJs see a Camelot wheel and semitone calculator, composers see chord changes and Roman numerals, and session musicians see Nashville numbering. Whatever your background, classical, jazz, pop, electronic, or you are still learning your first scale, KeyJ shows you music theory in the visual language you already understand. One Key, Every Musical Language Pick a key and scale once, and switch views any time: a lit up piano or fretboard, the Camelot Wheel, diatonic chords with Roman numeral analysis (RNA), or Nashville Numbering. It is all the same key, shown the way you think about it. Live Session: Play in Sync Start a Live Session, share a code, and join two or more devices into the same jam. When the band leader changes key, everyone following updates instantly. Great for a porch party, a masterclass where the instructor modulates and the class follows, or a jam session where half the room doesn't read charts. Following is free. Leading is a premium feature. Built In: Everything You Need * Camelot Wheel and Circle of Fifths for harmonic mixing, with adjacent compatible keys and major/minor slot at a glance * Semitone Converter: transpose by interval, or tempo fader percentage * Full key reference: key signature, relative major/minor, scale degrees, and diatonic chords * Roman numeral analysis (RNA) for major and minor keys * Nashville Numbering (premium) to follow a chart the way session players do * Instrument Mode: see any key on a piano keyboard or guitar fretboard * Advanced scales and modes: Dorian, Blues, Pentatonic, and more, with scale degree indicators on the carousel * Sharp/flat notation, major/minor filtering, and a growing set of themes including the free Midnight theme * Prevent Screen Lock to keep the display awake during a session * Long press the main screen for quick access to common settings Premium: More Power, Free to Try A single premium unlock adds Live Session leadership, advanced scale modes, Nashville Numbering, and exclusive Disco, Classical, and Cute themes. Try premium free for 10 days, and sample any premium theme up to three times before you buy. One purchase works on both iOS and macOS through App Store Universal Purchase. Built for Collaboration and Community KeyJ connects you to the JoyPunk community to share feedback, request features, and meet other musicians and DJs using the app.