21.2Global key versus local function

Suppose a tune is nominally in B♭ major.

That does not mean every chord should be analyzed as a Roman numeral in B♭.

If for two bars you hear:

Em7♭5 → A7 → Dm

then locally:

that is:

iiø–V–i in D minor

For those moments:

D is the temporary center.

This is more useful than forcing everything into B♭-major notation.