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Greensleeves is an English folk song originally  titled  "A New Northern Dittye of the Lady Greene Sleeves", published in 1580, from an unknown author (not King Arthur, most likely).
In 1865 William Chatterton Dix turned it into a Christmas Carol by adding the lyrics "What Child Is This".
In 2011 turned into a sparkling Balkan folk dance in 5/8 time signature, performed with a Greek baglamas.