Addressing a nun may seem intimidating at first; after all, they have devoted their entire life to the Catholic religion. However, you should not fear, because addressing a nun is the same as addressing any woman of faith in the Catholic religion. Use proper etiquette and show the nun that you are speaking with the same respect that you would show anyone else. Be polite and courteous, speak clearly, and you will have addressed the nun properly.
Addressing the nun in a face-to-face way is very simple and straightforward. If you do not know the woman of faith, refer to her simply as "Sister." If you know her name, refer to it after calling her "Sister" (e.g. Sister Mary Catherine).
Writing a letter to a nun may seem a bit trickier. Refer to the woman of faith by her name, followed by the abbreviation of the institution she belongs to (for example, the abbreviation of the Daughter's of Charity would be D.C., making the proper addressing in the written word: Sister Mary Catherine, D.C.).
Introducing a nun to someone else in a formal setting can be tricky as well. When introducing the nun, refer to her by her full name and the church she belongs to (e.g. Sister Mary Catherine of the Daughters of St. Paul). This will let everyone know her title (nun) her full name (Mary Catherine) and the ministry she belongs to (Daughters of St. Paul).
Addressing the Reverend nun is more complicated. When addressing them in a face-to-face setting you may refer to her in a few different ways. Reverend Mother Mary Catherine, Reverend Mother, or simply Your Reverence are all accepted addresses.
Addressing the Reverend in a letter is as simple as addressing a common nun in the written word. Simply refer to the Reverend by her title, full name and the abbreviation of the institution (e.g. The Reverend Mary Catherine, D.C.").
Addressing the Reverend in a formal setting is a bit tricky as well. Refer to the Reverend by her title (the Reverend Mother) her name (Mary Catherine, for example) and the organisation she resides over (The Daughters of St. Paul).