[Hawthorn]

>> Like ex-senators and former presidents, ex-governors retain their honorific titles <<

Not really accurate for former Presidents:

The correct etiquette has long been to call them by the highest titles they held before they were POTUS. So the proper way to refer to Bill Clinton is “Gov. Clinton,” and the proper way to refer to George Herbert Walker Bush is “Ambassador Bush.”

Granted that the USA’s decline of intellectual standards means almost nobody follows this proper etiquette anymore, but it’s still the way things should be done.

Still, “Gov. Palin” is entirely OK!

[Oztrich Boy]

The correct etiquette has long been to call them by the highest titles they held before they were POTUS. So the proper way to refer to Bill Clinton is “Gov. Clinton,” and the proper way to refer to George Herbert Walker Bush is “Ambassador Bush.”

Article I, Section 9, Clause 8:
No title of nobility shall be granted by the United States ...