Style of Peers' Widows
When a peer dies the only correct style for his widow is "the Dowager" prefaced to her peerage title. In cases where the new peer (in most cases, of course, a son) is unmarried, it seems to have become usual still to describe the lady as "the Duchess of " or "the Countess of", etc. There is no justification for this. All widows of peers are summoned to Coronations or other State occasions as Dowagers, and they are so addressed by the House of Lords.
If a modern leading case is required it is provided by Queen Mary. When King George V died and King Edward VIII succeeded unmarried, Queen Mary immediately ceased to be "the Queen", because that style and title is one which appertains only to a Queen regnant or the Consort of a King. In the ceremonial of the King's funeral she was described as Queen Mary and not as the Queen.
Thus the Countess of Blank cannot properly be so described when her husband dies and her unmarried son succeeds, because that style appertains to the wife of the "reigning" peer, and she does not fill that bill. No doubt some - probably a majority - continue to call themselves by the same style, but I know of no power which prevents someone calling himself or herself what he or she likes. But what they do must not be confused with what they may properly do.
In any case, as soon as the new peer marries, the widow must be distinguished in some way from the "reigning" peeress. As "the Dowager" is the correct form of that in which she would be summoned to any State function or be mentioned in the Court Circular, there should therefore be no objection to it. But undoubtedly the form is not popular with some ladies. It certainly does not convey an impression of youth, and that may be the explanation! Whatever the reason, there are some widows who prefer to prefix their former style with their Christian names - Arabella Lady This, Jane Countess of That, and so on. The mode seems to have become increasingly popular. It is necessarily adopted to avoid confusion when there is more than one Dowager.