A notebook for intervals, chords, scales, keys and harmony, showing each of them at once on a keyboard, a staff and a degree ruler. Nothing to install.
The board runs staff, keyboard, degree ruler from the top. The staff is the notes as they sound, the keyboard is which of the twelve you are using, the ruler is how far each note sits from the root. Seeing one thing three ways is what joins the theory to the sound.
The circle of fifths in the key panel turns. Whatever reaches the top becomes the current key, and the key signature, the scale and the diatonic chords all follow.
The progression strip in the harmony panel can be dragged into a new order. Press a chord to hear it, or play the strip through to hear it as a progression.
Chords and scales have a compare mode. Put two of them on the same ruler and you can see which single note moved, and how much that changes the character.