|From Debrett's Correct Form|
As a peer of the realm, the new Lord Snowdon will cease to be a commoner. He will sign his name as Snowdon. His wife, Lady Serena Stanhope,) becomes the Countess of Snowdon and will sign her name as Serena Snowdon.
I use Lady Serena because she is the daughter of the Earl of Harrington. At the time of her wedding in 1993, Serena was styled as the Hon. Serena Stanhope, as father, Viscount Petersham, did not succeed to the earldom until 2009. If Serena had married a mere Mr. Armstrong-Jones, she would have been styled. The. Hon. Mrs. Armstrong-Jones. This would have changed after her grandfather's death when she would have adopted the style The Lady Serena Armstrong-Jones. Mr. David and The Lady Serena Armstrong-Jones. No Mrs for daughters of dukes, marquesses or earls.
These are photos (no long lens for me at the time) I took of Lord Linley in New York City in the late 1980s, and Lord and Lady Linley at their wedding and in Washington, D.C., in 1993.