3.20This is why "the notes in the key" is not enough

Suppose someone says:

"This song is in C minor."

A beginner may expect only:

C D E♭ F G A♭ B♭

But you may routinely encounter B natural.

You might also encounter A natural.

Or other chromatic notes.

A key describes a tonal center and harmonic system, not an absolute prohibition on every other note.

This will become important when we start analyzing real music.