He had to improvise on the Hammerklavier when visiting Frederick the Great in Potsdam. That (improvising for Frederick) is also the starting point for the later

He had to improvise on the Hammerklavier when visiting Frederick the Great in Potsdam. That (improvising for Frederick) is also the starting point for the later creation of https://en.wikipedia.org/wiki/The_Musical_Offering https://en.wikipedia.org/wiki/The_Musical_Offering .