During the Baroque period, about 1600 AD and 1750 AD, music, for the first time, moved towards the major/minor system of tonality that we know today. The six “church modes” used in medieval and renaissance music (Ionian, Dorian, Phrygian, Lydian, Mixolydian, and Aeolian) were dropped in favour of just two; one with a major third above its root note (Ionian), and one with a minor third above its root (Aeolian). These became the Major and Minor keys that we still use today.
The baroque period also saw an increase in harmonic complexity, with a greater emphasis on counterpoint and ornamentation, as well as greater acceptance of chromaticism and some of the more dissonant harmonies.
One of the major developments in the Baroque was tuning systems that allowed instrumentalist to play in a major or minor key starting on any of the twelve notes of the chromatic scale (all the notes on the keyboard). Prior to this it was possible to modulate (change key within a piece of music) to only a small number of related keys. The new tuning systems (of which Equal Temperament is the standard today) gave composers much larger range of musical possibilities as they could now modulated from any one key to any other. J.S. Bach’s The Well Tempered Clavier was the first major work to fully explore the new tuning system.