This is the a chant of O Radix Jesse, translated into English by John Mason Neale for "The Hymnal Noted" in 1851. The calligraphy is closely based on BNF MS Lat 17436, a 9th-century Carolingian manuscript which is the earliest record of the "O Antiphons," of which O Radix Jesse is the third in the sequence.
The O Antiphons are traditionally chanted during Advent, from the 16th (or 17th) through the 23rd of December.
The carol "O Come O Come Emmanuel" is a paraphrase of the earlier O Antiphons.
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