Words at Play : 7 Downton Abbey Terms Americans Are Not Familiar With
The word lady is used in complex ways in British high society, where it usually corresponds to the use of lord for men. For example, it’s used when referring to women who hold certain titles: marchioness, countess, viscountess, or baroness. It can also be used of the wife of a lower-ranking noble, such as a baron, baronet, or knight.
Lady is also the courtesy title for the daughters of higher-ranking nobles: dukes, marquesses, or earls. The daughters of viscounts and barons are referred to as "The Honorable," and daughters of baronets or knights are simply called "Miss."