On a rare library visit I looked up the subject in a couple of etiquette guides.
Debrett’s concentrates on the formal, though formal means “proper” rather than just “professional”. The second book is based on business etiquette and has chapters on England (I didn’t check for Scotland, sorry) and the US. This was the only book I could find with anything on US etiquette.
From Debrett’s Correct Form:
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“The recipient of a doctorate conferred by a university … is entitled to be addressed as ‘Doctor’.”
“The recipient of an honorary doctorate is entitled to the style of ‘Doctor’”.
“It is a matter of choice whether the appropriate degree(s) should be placed after his (or her) name or to address the Doctor as Dr. …It is, however, recommended that the letters, where known, are used to distinguish him (or her)from a medical practitioner who is known as ‘Doctor’, whether or not he is a doctor of medicine”
“The title [of doctor] has become so wedded to the medical profession that the reminder is needed that not all qualified medical men hold the final degree…Those medical men who have not taken their final degree are addressed in every way as if they have, excepting that their actual qualifications, such as M.B., L.R.C.P., would be substituted for M.D. on the envelope, or, as is nomal with general practitioners, they are called simply Dr. -----”**
From “Kiss, Bow or Shake Hands: How to do Business in Sixty Countries”, Morrison, Conway, and Borden
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US:
“To show respect, use a title such as Dr., Ms., Miss, Mrs., or Mr. with the last name.”
“When you meet someone for the first time, use a title and their last name until you are told to do otherwise”
England:
“Rather than ‘sir’, you should use the title of the person you are addressing”**
My impression is that it is ok to call PhD’s “Doctor”, but it is more common to add their last name. I also see I have been using MD incorrectly to refer to and MB or equivalent when it should be reserved for a Doctor of Medicine (i.e. a higher degree). Wonderful books, both of them, full of useful gems.
BTW, Katie, yes, it is a Big Knights ref., I guess I have a thing about titles