"Viderunt Omnes" is a traditional Gregorian chant of the 11th century. The work is based on an ancient gradual of the same title. The chant was subsequently expanded upon by composers of the Notre Dame school who developed it as type of early polyphony known as organum. Thought to be written for Christmas, the polyphonic settings would have retained the same liturgical purpose as the original gradual, while being musically enhanced for the festivities. The cantus firmus, or tenor, "holds" the original chant, while the other parts develop complex melismas on the vowels. The various settings of Viderunt Omnes provide context for specific trends in medieval music.