The Byzantine Empire
Late Antiquity
Hbomber, CC BY-SA 4.0 , via Wikimedia Commons
It is important to remember that the "Fall of Rome" and the dawn of the "Dark Ages" refer only to the Roman Empire's loss of control over its Western provinces. In the East, the Roman Empire continued not only to exist, but remained a global superpower, albeit no longer an invincible one. However, three major changes gradually made the Roman Empire a very different place than the one Augustus would have recognized.
First, the importance of the city of Rome had become symbolic rather than strategic, especially after Constantine had built his new capital at Byzantium. By the time the Western provinces were lost, Constantinople had long eclipsed Rome in political importance. Second, due to the gravity of the East, Greek soon replaced Latin as the language of the Romans. Finally, Christianity went from being a persecuted minority to the Empire's official religion.
Gaspare Fossati (1809-1883), Public domain, via Wikimedia Commons
To distinguish the Latin-speaking pagan empire with its capital at Rome from the Greek-speaking Christian empire with its capital at Constantinople, many historians refer to the former as the "Roman Empire" and the latter as the "Byzantine Empire." The "Byzantines" themselves, however, recognized no such distinction, and continued to refer to themselves as "Romans" for another thousand years.
In the sixth century, Emperor Justinian built an enormous church called the Hagia Sophia, Greek for "Holy Wisdom." A masterpiece of engineering, it was one of the largest indoor spaces in the world when completed. The interior acoustics of the building allowed for the propogation of sound waves in a way never before heard.
The music that developed for liturgical rites in the Hagia Sophia is known as Byzantine Chant. This music is distinguished from the purely monophonic music that preceded it by the use of a long drone called an ison, which is a tone sustained beneath a melody that moves around above it. As the pitches of the melody change, each produces a different harmony against the ison. The ison will occasionally change pitch to create a new set of harmonies.