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How can a key signature identify a relative major or minor key?

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scale The universality and innate nature of music, and the pentatonic scale, as demonstrated by singer Bobby McFerrin.

A key signature identifies both a major key and its relative minor key because they share the exact same set of sharps or flats. In music theory, the circle of fifths serves as the primary tool for visualizing this relationship.12

The circle is organized with two layers: the outer layer represents major keys, while the inner layer represents their corresponding relative minor keys. Because they share the same key signature, they occupy the same position on the circle. For example:12

No sharps or flats: C major and A minor1
One sharp: G major and E minor1
One flat: F major and D minor1
Two sharps: D major and B minor1
Two flats: B♭ major and G minor1

By observing the number of sharps or flats at the beginning of the staff, a musician can identify the potential major key and its relative minor counterpart simultaneously. The specific tonic note of the piece—the central pitch around which the scale is built—ultimately determines whether the music is functioning in the major or relative minor key.12