Why is Greensleeves and What Child is This the same?

"Greensleeves" and "What Child Is This" are the same in that the latter is a Christmas carol that uses the melody of the former. "Greensleeves," a traditional English folk song, dates back to the 16th century, while "What Child Is This" was written by William Chatterton Dix in the 19th century, with lyrics reflecting the nativity theme. The combination of the well-known melody with new lyrics has made it a beloved holiday song.