Who did introduce the mediant keys in to music harmony progression?

Question

Which composer(s) introduced as the first time mediant chords and mediant keys in their compositions - or in which period the use of this kind of modulation was coming up?

Answer 1

Carlo Gesualdo was one of the first composers to explore unusually chromatic modulations and chord changes. However, this was the late Renaissance era when composers didn't think of music in terms of chord progressions or keys the way we do now. Chords would arise out of the voice-leading and the counterpoint that the melodic lines created.

In the baroque era, once the major and minor scales as we know them became the norm, most pieces in a minor key would move to the relative major (III) at some point in the piece, and pieces in a major key could move to it's relative minor (the submediant). Haydn and Mozart also explored tonicizing the chromatic submediant (bVI) in major key pieces by lingering on the harmony of a deceptive cadence. They could also modulate to any key they wished in the development section. But it wasn't until around Beethoven that non-diatonic mediants and submediants became major structural key centers of a composition. Check out his Hammerklavier Sonata which modulates from Bb major to G major in the exposition. There's a good amount of literature out there about Beethoven's exploration of mediant harmony and how it influenced composers of the romantic era, (especially Schubert). Hope this helps!