12.7Notice all the “wrong notes”

In C major:

C D E F G A B

But secondary dominants introduce chromatic notes:

A7 = A C♯ E G

C♯ is outside C major.

B7 = B D♯ F♯ A

D♯ and F♯ are outside.

C7 = C E G B♭

B♭ is outside.

D7 = D F♯ A C

F♯ is outside.

E7 = E G♯ B D

G♯ is outside.

Those notes are not random.

Each one creates a leading-tone or dominant relationship to the destination chord.