The term 'letter ending' refers to the way a letter is concluded. The two most common letter endings are 'SINCERELY' and 'YOURS'. 'SINCERELY' is a nine-letter word that is used to express sincerity or earnestness. It is often used in formal letters, such as business correspondence. 'YOURS' is a five-letter word that is used to express affection or respect. It is often used in informal letters, such as those sent to family and friends.