My Lord Duke

First Duke of Wellington

Loretta reports:

When a commenter on my Footman post noted the writer’s puzzling use of the term “My Lord Duke,” I assumed this was a case of either ignorance or disrespect.  This was because I’d learned that a duke is addressed as “Duke” by persons of rank and “Your Grace” by members of the lower orders.  He’s never “Lord So-and-So” or “Your Lordship” or “My Lord.”

“My Lord Duke,” however, is another matter.

Neither Manners and Rules of Good Society nor Whitaker’s Peerage mentions the term, but further sleuthing quickly produced it.  Unlike lower grades of the peerage, according to my vintage (1978) copy of Debrett’s Correct Form,  in conversation, “a Duke is always so described,” i.e., he’s never referred to as Lord So-and-So.   But he can be “My Lord Duke," as in the following examples:

BEGINNING OF LETTER
Formal    My Lord Duke
Social        Dear Duke
        Dear Duke of Hamilton may be used if the acquaintanceship is slight

But note:

VERBAL ADDRESS
Formal        Your Grace
Social            Duke
Employee status    Your Grace

Note, too, the difference between addressing royal and not-royal dukes in this excerpt from Blackie's Modern Cyclopedia Of Universal Information, Vol 1, 1890:
 
A royal duke should be addressed as Sir, not My Lord Duke; and referred to as Your Royal Highness . . .
Duke and Ducal Family.—His Grace the Duke of—- ;My Lord Duke, Your Grace. Her Grace the Duchess of —-;Madam, Your Grace.
Here’s one of many examples from the Despatches, Correspondence and Memoranda of Field Marshal Arthur Duke of Wellington, Volume 1, 1867:

Sir W. Congreve to Field Marshal the Duke of Wellington.
My Lord Duke,                                       13, Cecil Street, London, 9th Aug., 1822.
     Having had the honour of exhibiting to you the accuracy of direction to which the rockets are now brought, and the facility of manœuvring and bringing them into action, one point only remains for demonstration, which is not in itself so apparent to the observer as those above alluded to, and which I therefore think it my duty to take this mode of stating to your Grace.

Letter writing advice from The Christian's Accomptant, 1831, advises:

To His Grace the Duke of S—- ,
My Lord Duke, or May it please your Grace.

I hope that clears everything up.