Plainchant is invented.
Organum, the predecessor to polyphony is invented in France.
Monteverdi composes the first opera Orfeo in 1607.
The reformation. Vocal religious music enters the vernacular in the places that convert to protestantism. Examples include Ein Fest Burg ist Unser Gott
The invention of equal temperament. The Well Tempered Clavier was written to show off what equal temperament could do.
The decline of the highly complicated baroque form in favour of the simpler, more tuneful and easier to understand ["style galante"] of Haydn, J.S. Bach's sons and Mozart during his early years.
Haydn inventing the symphony, the most ubiquitous musical form for centuries. Haydn symphony no.45 F-Sharp Minor
Beethoven composes more complicated symphonies than ever before, pushing the boundaries of symphonic and sonata form and setting a standard that every other composer ever had to live up to. Symphony no. 9 and Symphony no 3 E Flat Major
Berlioz completes his "treatise on orchestration". Although it can be difficult to see in youtube videos check out his Symphonie Fantastique
Liszt brings the symphonic poem to prominence. Les Preludes. He also used Thematic transformation, where a theme or leitmotif is transformed and does not bear much resemblance to the original theme and now stands on it's own without the theme.
The rise of Romantic Nationalism has huge influence on composers such as Dvorak, Janacek, Smetana, Sibelius, Vaughn-Williams, Holst and on the Russian composers known as "the five". Quite often their compositions include folk melodies as a way of representing a nation or people.
Wagner begins the march towards atonalism and modernism in his later operas.
Schoenberg comes up with serialism and the twelve tone system. eg. Schoenberg Piano Concerto and the Requiem Canticles by Stravinsky.
Late Romanticism from the likes of Mahler, Rachmaninov and Shostakovitch stand in contrast to serialism.
Minimalism such as Glass Symphony 8 is characterised by repeating motifs such as the above Glass or just repeating notes such as In C by Terry Riley to no sound at all such as 4'33"
Finally we reach now, and I have no idea what is going on. I suppose you could call it post modernism. It's not minimalism, not pop, not modernism. Anyway, we have folks like Einaudi
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