What do we call the minor key that shares the same tonic as a major key but has a different key signature?

The minor key that shares the same tonic as a major key but has a different key signature is referred to as the parallel minor. This relationship is defined by the tonic note being identical in both the major and minor keys, but their tonal characteristics differ significantly due to the alteration of other scale degrees.

For example, C major and C minor share the same tonic note, which is C, but their key signatures differ—C major has no sharps or flats, while C minor has three flats. The parallel minor will have a scale that starts with the same tonality (in this case, C), but the intervals and overall mood of the scale differ due to the altered notes in the minor context.

In contrast, the relative minor, which also shares the same tonic, is determined based on the major scale’s sixth degree. This relationship does not maintain the tonic but instead shifts to a different starting point entirely while maintaining a similar key signature, leading to different implications in harmony and melody. Other options like the melodic and chromatic minor refer to specific alterations made within the minor scale but do not align with the concept of sharing a tonic with a major key.