Officially, the widow of a duke is known as “The Dowager Duchess of …..”
However if there is already a dowager duchess in that family still living then the widow of the junior of the dukes takes her forename and places it in front of the title. For example, the widow of the senior duke would be Dowager Duchess of Stepney and the widow of the junior duke would be Susan, Duchess of Stepney.
But, nowadays many of the widows prefer to put their forename instead of Dowager.
Things are never simple though – in the case where the duke is not married his widowed mother does not become the dowager duchess (nor does she use her forename in front of her title) she maintains her original title of The Duchess of Stepney.
How to address the Widow of a Duke:
Usually, in a social situation, you would address her as follows:
If you are writing a letter you would start with “Dear Duchess” and end with “Yours sincerely”. On the envelope you would write “The Dowager (or “Susan”, if she is not the eldest widow) Duchess of Stepney”
When speaking to her you would call her “Duchess”.
A lunch invitation would be sent to “The Dowager (or Susan) Duchess of Stepney”
If you refer to her in conversation to someone you would say “The Duchess of Stepney”.
The place card for the table would read “The Dowager (or Susan) Duchess of Stepney”
And a legal document would call her “The Most Noble Susan Mary Victoria Duchess of Stepney”.