This is the third grade in the peerage. His Royal Highness The Earl of Wessex is a member of the Royal Family and should be addressed as such (The Earl of Wessex).
In conversation, an earl is referred to as Lord (Aldford) rather than the Earl of (Aldford).
It should be noted that although most peers of this rank are Earls ‘of’ Somewhere, there is a significant number that are not. The following titles are prefixed by ‘Earl’ not ‘Earl of’: Alexander of Tunis, Annesley, Attlee, Baldwin of Bewdley, Bathurst, Beatty, Belmore, Cadogan, Cairns, Castle Stewart, Cathcart, Cawdor, Cowley, De La Warr, Ferrers, Fortescue, Granville, Grey, Haig, Howe, Jellicoe, Kitchener of Khartoum, Lloyd George of Dwyfor, Mountbatten of Burma, Nelson, Peel, Russell, St Aldwyn, Spencer, Temple of Stowe, Waldegrave and Winterton.
Ecclesiastical, ambassadorial and armed forces ranks precede an earl’s rank in correspondence. For example, Major-General the Earl of (Aldford)’.
When an earl is also a privy counsellor or has received a knighthood he has the appropriate post-nominal letters.
A number of earldoms can be inherited in the female line and a countess in her own right would be addressed as for the wife of an earl. Her husband derives no title or style from his wife.
The wife of an earl is a countess and is known as Lady (Aldford). Use of the title countess in speech is socially incorrect unless it needs to be specifically mentioned, for example in a formal introduction. The sole exception to this is HRH the Countess of Wessex who is always referred to as ‘Countess’.
In official documents the style of The Right Hon should still be used for both an earl and countess.