Wolfgang Amadeus Mozart
Some time after Mozart’s death a man called Köchel studied all Mozart’s music, tried to put them in chronological order, and gave them a number. The number helps us to know exactly which work is meant, for example Symphony in G minor K183 is not the same piece as Symphony in G minor K550 (K stands for Köchel. Sometimes it is written "KV550" standing for "Köchel Verzeichnis" i.e. "Köchel Catalogue"). The highest Köchel number is 626, his requiem mass.