Guido of Arezzo's "Micrologos"This piece Introduced four major ideas to musical transcription as well as playing:
1. A 4 line staff
2. Relative Pitch
3. Sight Singing Syllables
4. Indication of whether notes are flat or natural (rounded B meant flat, square B meant natural)
Hildegard of Bingen
Ars Nova TreatiseThe title meaning "New Art" as opposed to 'Ars Antiqua' which means "Old Art", this is the period in time (especially in France) where music began to prosper and become more publically understandable.
During this period, we saw innovations such as understanding pitch intervals, as well as the ability to indicate time in sheet music.
Josquin's "Ave Maria...virgo serena" Motet
Martin Luther Chorale "Ein feste burg (A Mighty Fortress...)"
Arcadelt Madrigal "Il bianco e dolce cigno"
Palestrina "Pope Marcellus" MassDedicated to Pope Marcellus, by legend this piece was among the first to show that with 6 voices, sacred words could be intelligible.
Victoria "Missa O magnum mysterium"
Gabrieli "Sonata pian'e forte"Prior to Gabrieli's sonata, you would have someone who sings in whatever range perform whichever part (between treble, alto, tenor, bass etc.). This piece (written in Venice, Italy) was the first in history to indicate which specific instrument is meant to play each part.
This is also the first piece in history to indicate any dynamic contrast. Prior to this piece being written, a full song was either loud or soft from beginning to end, there were no indicative markings to get louder or softer.