From: https://www.biograview.com/portfolio_page/music-in-the-baroque-era

Medieval music which lasted from 500-1400 A.D., was the earliest recognised era of music. The Gregorian Chant, otherwise known as plainchant or plainsong, was created between 590 to 604 A.D. by Saint Gregory who was the first Roman Catholic Pope. The Gregorian Chant was oral music learned through repetition before music notation was established. It was also monophonic, meaning there was only ever one melody being sung at a time. There was a focus on singing rather than on instrumental music during the Medieval era, possibly because the Gregorian Chant was incredibly sacred. After Guido D’Arezzo created a form of notation between 1025 and 1030 A.D., plainsong was able to be written down in the same music notation we have now, using a four-line staff and clefs.

Chant formed the basis of early musical expression and formal composition.  This chant by Guido D'arezzo forms the basis of organised scalic pitches.  Based on an acrostic which forms D'arezzo's Prayer to Saint John the Baptist, we get the earliest pitch names (solfa):

Particular chants develop and are used regularly for particular parts of the liturgy. These 'fixed' compositions became known as cantus firmus:

From: https://study.com/academy/lesson/cantus-firmus-definition-examples.html

Cantus firmus, Latin for ''fixed song'', is a melody derived from preexisting sacred plainchant, used as a tool to create melodic counterpoint between the known chant tune and a secondary melody. Cantus firmus was most popularly used in sacred polyphonic compositions well into the Baroque Era of classical music, especially with cantus firmus mass. A mass is a large sacred body of work that outlines the Catholic Church liturgy, and it was often written with cantus firmus.

The plainchant melody, also called the vox principalis or ''chant voice'', would typically be extended and stretched to longer sustained rhythmic values, while the duplum or second voice would perform above it. Imagine the cantus firmus definition in exact opposition to the homophonic chordal accompaniment in modern pop music or opera. Instead of the primary melody being accompanied by the harmony, the melody itself becomes a type of background accompaniment as a cantus firmus line.

One such chant that has an enduring impact on Music throughout the ages is the Dies Irae (Day of Wrath), which forms part of the Requiem Mass.