Four voices — soprano, alto, tenor, bass — moving from chord to chord under the rules of common-practice part-writing. Move any voice with the arrows and this checks your work in real time, the way a theory teacher would: it flags parallel fifths and octaves, hidden fifths/octaves into the outer voices, voice crossing and overlap, spacing over an octave, and a doubled leading tone. The starting example has deliberate errors — fix them until it's clean, and play it as you go. (Voices move through the C-major scale, so spellings stay correct.)