Number 38 in the countdown of classical music is a version of a 13th century song that was written by Wolfgang Amadeus Mozart (1756-1791). It is called Ave Verum Corpus.
According to wikipedia.com, “Ave verum corpus” is a short Eucharistic chant that has been set to music by many composers. It dates to the 13th century, first recorded in a central Italian Franciscan manuscript.
Musical settings include Mozart‘s motet Ave verum corpus (K. 618), as well as settings by William Byrd and Sir Edward Elgar. Not all composers set the whole text. For example, Mozart’s setting finishes with “in mortis examine”, Elgar’s with “fili Mariae”. Marc-Antoine Charpentier composed three versions: H.233, H.266, H.329.
Mozart’s version, with instruments only, was adapted by Pyotr Ilyich Tchaikovsky as one of the sections of his Mozartiana, a tribute to Mozart. From the 21st century there are settings by the Swedish composer Fredrik Sixten and the English composer Philip Stopford.
You learn something new every day. Hopefully.