in the diatonic major-minor system, two keys of opposite modes that have the same scale (identical key signatures). The tonic triads of relative keys include the same major third.
Relative keys are very closely connected. Because of their common scale structure, they can be united (in the Russian folk song, for example) into a relative peremennyi lad (a mode in which the function of the tonic shifts from one tone to another, or in which the scale changes but retains the same tonic). As a result of the development of the harmonic system, the correlation of relative keys is becoming more complicated in contemporary music, particularly within the framework of the combined major-minor and the chromatic system.