These letters from the Duchess of Devonshire largely concern the marriage of her son Lord Hartington to Lady Charlotte Boyle and her objection to it which caused a rupture in the Devonshire family relations between the Duke and Duchess, the Duke being supportive of the match. The letters include one that was drafted [according to a note by Frederick Cavendish] by the 3rd Duke from the Duchess wishing Lord Hartington happiness in his future (CS1/344/0).
The letter to the 3rd Duke specifically concerns her need to tell the 3rd Duke how she is feeling about Lord Hartington's match but finding it impossible to speak to him in person about it. She writes that she wishes for him to give her leave to part company and live separately as they cannot live in friendship after their differing opinions of Lord Hartington's match. She writes of her love for her children and her duty to them and him by raising them to have duty and regard for their father but in doing so they have now little for her after the disagreement between them about Lord Hartington's match. She adds that her affection for the 3rd Duke remains but that she cannot live with him after this rupture and mentions Lady Abercorn's attempt to persuade her of the match.
In her letters to Lord Hartington she writes that she agrees marriage is a most important part in life in determining one's happiness or sadness. She adds that she believe happiness was not a factor of his decision or he would not have chosen "a child for a companion". She rejects the rumour that she is unhappy about the match because of the other potential girl, who she had never met before. She also asserts that her removing herself from company was due to her presence making the 3rd Duke angry and preferring to manage her melancholic feelings by being alone and not having to make conversation. She writes to tell him of her letter to his father asking that she may remain in the country rather than return to London.
There is also a letter to Lady Burlington in which the Duchess passes on a message from her very ill husband (who she is now reconciled with) that they will dine at Chiswick if it can be without ceremony (CS1/344/4), written a fortnight before the 3rd Duke dies.