Final answer:
The musical staff was developed in the Middle Ages in Italy by Guido D'Arezzo and is still used today to write and read music.
Step-by-step explanation:
The important thing that was developed in the Middle Ages in Italy by Guido D'Arezzo and is still used today is the musical staff. Guido D'Arezzo was a music theorist who created a system for writing music using a set of lines and spaces, which we now know as the musical staff. This invention allowed musicians to visually represent pitch and rhythm, making it easier to read and write music.
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