Notating Minor Key Signatures

Every minor key has one key signature, and it belongs to the natural minor form. Natural minor uses no altered scale degrees beyond whatever sharps or flats sit in that key's signature, so its pattern of sharps or flats is fixed and stable. Harmonic minor raises the seventh degree, and melodic minor raises both the sixth and seventh going up, but those raised notes never get folded into the signature itself. Instead, a composer writes them in as accidentals, note by note, wherever the harmonic or melodic form is actually used.

This is why G minor, whose natural form has B flat and E flat, keeps only those two flats in its signature even in a passage built on harmonic minor. The raised seventh degree, F sharp, shows up as an accidental in front of the F each time it's needed, not as a standing sharp at the clef. The logic matters because a key signature is a promise about every occurrence of a pitch letter for the whole piece or section, and the raised degrees in harmonic and melodic minor are occasional, expressive choices, not permanent features of the key.

Minor key signatures connect directly to relative majors: every minor key shares its exact signature with the major key built on its relative, a minor third above. That shared signature is also why the circle of fifths works the same way for minor keys as for major ones, just centered on a different tonic. Once a signature is set from the natural form, everything else, harmonic minor's accidental, melodic minor's two accidentals, builds on top of it without changing it.