In the novel Lord Edgware Dies, Mrs Dortheimer and her husband are extremely wealthy and extremely musical. They have a box in Covent Garden. They invite young men with prospects to the box.
Ronald Marsh was invited, and although he doesn't like opera he likes dinners at Grosvenor Square. On the night Lord Edgware was murdered he first dined with the Dortheimers in their home, then they went to the opera and after that they had supper at Sobranis.