Some musicologists (people who study and write about music) have studied all the works by a famous composer and have given them a catalogue number. For example Mozart’s music does not have opus numbers. Some of them are long operas, others are tiny little pieces for the piano he might have written in a hurry one day. A man called made a list of every single work by Mozart and gave them K numbers (K for Köchel). His numbering goes up to 622. This is useful, for example, to tell the difference between his Symphony in G minor K183 and his Symphony in G minor K550.