Opus numbers have been used inconsistently throughout history, they are assigned by composers to their compositions, they often are incomplete and only roughly indicate a chronological order. Moreover, in case of Strawinsky, the vast amount of variations and derivations of his works require an overall different system to organize a catalog. Several attempts have been made before Kirchmeyer came up with his annotated catalog that is now widely accepted, and K numbers provide a helpful shorthand to refer to Strawinsky’s works.