The same question was asked in the Q&A section on the website for
The Protocol School of Washington's Honor & Respect: The Official Guide to Names, Titles, and Forms of Address:
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QUESTION-
Addressing A Former President
I will meet President ******* in a few weeks in person. I know they call former US Presidents "President *******." But what would I call him when I meet him or when I introduce others to him: e.g. "Mr. *******", or "President *******? Thanks for your help.
-- Politico, Clarendon Hills, IL
ANSWER-
Hi Politico:
Former Presidents are directly addressed as "Mr. (Name)", not as "President (name)", and they are identified as "the former President of the United States".
You will hear the media say "President *******." BUT in direct address it is not correct. "President *******" and "President Bush" are used on TV so the listener can be clear who is being discussed. But
in direct address it's "Mr. President" for the current president, and "Mr. (surname)" for former presidents of the United States.
"The Honorable William Jefferson *******" .... is correct when you are using the FULL NAME in writing because once elected, officials are addresses as "The Honorable" for life. And that's the correct form for formal introduction -- like from a podium before his speech to the audience -- but you wouldn't address him that way in conversation.
-- Robert Hickey
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The Protocol School of Washington is probably the most well respected protocol and etiquette school in the United States. The school's deputy director Robert Hickey is the author of the book
Honor & Respect: The Official Guide to Names, Titles, and Forms of Address. Many people consider this book to be the bible of protocol/etiquette.
Even though most people think it sounds more respectful to directly address the former president as Mr. President, it's not correct. But no one is going to correct you if you call him Mr. President because 1. it would be rude to do so and 2. protocol training has become an anachronism. The vast majority of us don't receive protocol training anymore and no one truly expects us to follow it anymore. But there
is a "rule" even if we choose not to follow it.