You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Dwight_D._Eisenhower

Context:
Since the 19th century, many if not all presidents were assisted by a central figure or "gatekeeper", sometimes described as the President's Private Secretary, sometimes with no official title at all. Eisenhower formalized this role, introducing the office of White House Chief of Staff – an idea he borrowed from the United States Army. Every president after Lyndon Johnson has also appointed staff to this position. Initially, Gerald Ford and Jimmy Carter tried to operate without a chief of staff, but each eventually appointed one.

Question:
What 19th century informal political office did the modern role of White House Chief of Staff correspond to?

President's Private Secretary