Giving pieces of music opus numbers helps us to identify which piece of music (from a certain composer) that composition is. For example: Beethoven wrote lots of piano sonatas. His first Piano sonata in A flat major has the opus number of op.26. This shows that he wrote this sonata when he was young in his composing career. Many years later, he wrote another piano sonata which is also in A flat major, and this piece one has the opus number of 110 (op. 110).