What does k626 mean?

K626 refers to Wolfgang Amadeus Mozart's "Requiem in D minor, K. 626," which is one of his most famous and unfinished works. Composed in 1791, it was commissioned by an anonymous patron and completed posthumously by Franz Xaver Süssmayr. The piece is notable for its emotional depth and complexity, often performed in memorial services and classical concerts. The "K" stands for Köchel, the cataloging system used to organize Mozart's compositions.