The triad over root principle can also be used to create 9th chords by placing a different root note under a 7th chord as shown here. By placing an Em7 chord over a C bass note, a CMaj9 chord is created.
Like triads, this process works for every note of the scale and can therefore be applied to scale tone 7th chords in any key. The following example shows the scale tone 7th triad over root substitutions for chords II, V and I in the key of C.