Guido d’Arezzo is the person given the credit for inventing modern music notation that is widely followed. His two most famous creations are the staff, aka the five bars the notes are written on so you know which notes they are. I think each “square” is then divided up into a time, like 3/4 or 4/4. He also came up with the mnemonic of do re mi but a couple of the words were different because they’re taken from a hymnal in Latin. Someone else modified them for English speakers.
Guido was a Benedictine monk who developed these musical innovations to help others learn Gregorian chants quickly. There had been other systems before him yet I guess they didn’t work well. His technique was so successful it made him famous throughout what is now northern Italy. However, Guido drew the ire of others in his abbey (even monks have feelings despite all the religious training in humility) which led him to relocate to Arezzo at the invitation of Bishop Teldad. The town didn’t have a monastery but there was a cathedral with a large group of singers to train.
During his years in Arezzo he wrote everything down in a book called Micrologus around 1025 or 1026 AD. This foundation has been continued to this very day.
Sadly, not much else is known about the monk after 1028 besides him going to Rome to show his technique to Pope John XIX.
Another legacy is his first name (GUIDO) being a computer notation format for music. You can check it out at this Web site.