The hierarchy of Catholic Church leadership can be confusing to those unfamiliar with its intricacies. Clergy are accorded specific titles of address based on their ranking. Cardinals and monsignors are similar in that both titles are honorary and are given to clergy members of extremely distinguished service. A cardinal is typically a bishop or archbishop who has been selected to serve on the committee that appoints the new pope, and a monsignor is a diocesan priest who has served both his diocese (a district or region) and supervising bishop with faithfulness and reverence.
Refer to a cardinal in introductions and conversations as "Your Eminence," especially if you are meeting the cardinal for the first time. As time passes, it is acceptable to address the cardinal by his title and last name ("Cardinal Smith"). This holds true in all Roman Catholic societies except the church in Great Britain, where a cardinal is addressed as "Your Grace." Since a monsignor is lower in the church hierarchy than a cardinal, a less formal salutation is used. It is acceptable to introduce and address a monsignor by his title and last name ("Monsignor Brown").
Address letters to cardinals by writing "His Eminence/Grace" (depending on the country) the cardinal's first name, title, last name and diocese on the outer envelope. The finished name on the address will read "His Grace, Thomas Cardinal Wolsey, Archbishop of York." When addressing a letter to a monsignor, however, the title is slightly different. You may address the letter to either "The Very Right Reverend Monsignor Brown" or to "The Reverend Monsignor Brown." There is no hard and fast rule, though you may wish to reserve the more formal "The Very Right Reverend..." for a monsignor you do not know very well.
Start a letter to a cardinal with the proper salutation. "Most Eminent Cardinal" is the accepted form of salutation in letters to British cardinals, while "Most Reverend Eminence" is used in other Roman Catholic countries. "Your Eminence" is an acceptable salutation if you have known the cardinal for many years, though the familiar British address is "My Lord Cardinal." Monsignors may simply be addressed with "Dear Monsignor."
No one will be offended if you offer more respect instead of less. If you are not sure exactly how to address a clergy member, err on the side of caution and use the most formal style address for each rank. Present guests to the clergy member you are introducing as a sign of respect for the office. Do not present the clergy to the guests. When you enter a meeting with a cardinal, remember that it is a sign of respect to kneel on the left knee when making your first address and bowing over his hand. Repeat this gesture upon leaving the cardinal's company. You are not required to kneel in the presence of a monsignor. Men must remove their hats when addressing any member of the clergy.