The chord factory

Every key ships with the same seven chords

Stack a third and another third on each note of the major scale and seven chords fall out — and their qualities are always, in every key, major, minor, minor, major, major, minor, diminished. That invariance is why Roman numerals work: I–V–vi–IV names the positions, and the positions never change. Slide the key and watch the letters change while the pattern refuses to.

Tap any chord to hear it. The quality badges never move.

Build a progression — then move it

Fill the four slots with numerals, play the loop, then change the key and play it again: the progression survives the move untouched. That is what numerals are for.