KeyJ: Music Theory

KeyJ is the music theory companion built for DJs, musicians, producers, and students who want to understand keys, chords, and scales without cracking open a textbook. Whether you are warming up for a jam session, building a setlist, or studying harmony for the first time, KeyJ puts the essential information in your hands in seconds.

For DJs: Harmonic Mixing Made Simple

Knowing which key is playing next is the difference between a smooth transition and a clashing disaster. KeyJ includes full support for the Camelot Wheel, the industry-standard reference used by DJs worldwide for harmonic mixing. Select any key and instantly see its Camelot position, its adjacent compatible keys, and whether you are in a major (B) or minor (A) slot. No more memorizing charts or second-guessing your next track.

For Musicians and Producers: A Complete Key Reference

Every key in KeyJ opens a full theory breakdown. You get the key signature with the exact number of sharps or flats, the relative major or minor key, and a complete scale degree table showing all seven notes alongside their diatonic chords. Whether you are working out a chord progression, finding chords that naturally belong together, or looking for a pivot point to modulate, it is all laid out clearly.

Roman numeral analysis (RNA) labels each degree the way theory is taught: I, ii, iii, IV, V, vi, vii° for major keys, and i, ii°, III, iv, v, VI, VII for minor keys. This is the language musicians and educators use when talking about chord function, and KeyJ uses it consistently so you can apply what you learn directly to real musical situations.

For Students: Learn Theory by Exploring

One of the best ways to understand music theory is to explore relationships between keys. KeyJ lets you do exactly that. Swipe through all 12 major or 12 minor keys on an interactive carousel, watch the scale degrees and diatonic chords update in real time, and start recognizing the patterns that make tonal music work. Toggle between the Circle of Fifths and the Camelot Wheel to see two different but equally valid ways of organizing the same 12 keys. Compare a key to its relative major or minor and watch how the sharps and flats shift as you move around the cycle.

This kind of active exploration builds intuition faster than passive study. KeyJ supports that process at any skill level, from a first encounter with key signatures to a working musician who needs a fast reference on stage.

Customizable for the Way You Work

KeyJ adapts to your preferences. Switch between sharp and flat notation depending on the key you are working in. Toggle between major and minor modes to filter the carousel to only the key type you need. Choose light or dark theme for your environment. Every setting persists between sessions so the app remembers how you like to work.

The Circle of Fifths and the Camelot Wheel, Side by Side

These two systems represent the same harmonic relationships from different perspectives. The Circle of Fifths organizes keys by the interval of a perfect fifth and groups relative major and minor pairs. The Camelot Wheel maps that same structure onto a numbered clock face optimized for DJs who need quick visual reference. KeyJ lets you move between them freely, making it useful whether your background is classical, jazz, pop, electronic, or anything in between.

Built for Collaboration and Jamming

Music is more fun with other people, and KeyJ is built with that in mind. The same key reference that helps you understand a chord progression also helps you communicate it to bandmates, collaborators, and students. When everyone in the room can quickly check what key they are in and what chords belong there, rehearsals move faster and jams go deeper. KeyJ also connects you to the JoyPunk community to share feedback, request features, and meet other musicians and DJs using the app.

Available on iPhone, iPad, Android, and macOS. Free to download. A premium version is coming soon.