The simplest key in music — seven chords, no accidentals.
See how all seven chords sit together, and hear what makes each degree do its job.
Open C Major in the Studio →The key of C Major contains C, Dm, Em, F, G, Am, Bdim. In Roman numerals that's I, ii, iii, IV, V, vi, vii° — the tonic (C) is home, and the dominant (G) creates the tension that resolves back to it.
Because it has no sharps or flats, so every note is a white key on a piano and every chord is a plain, open shape on guitar. That makes the relationship between the scale and its chords as visible as it ever gets — which is why theory is almost always taught from C.