If you are writing a business letter, "Re:" below the address of the recipient at the top of the letter lets the recipient know the subject of your letter. It stands for "regarding." You can also use "CC" at the top of the letter to let the recipient know that you are sending the letter to more than one person. It means "carbon copy."
Use two letter abbreviations if you want to write the name of a state as an abbreviation. Do not put periods between the letters. For example, write, "TX" for Texas. When you abbreviate the names of streets, write "St." for "Street," "Ave." for "Avenue," "Dr." for "Drive," "Blvd." for "Boulevard" and "Pkwy" for "Parkway."
"Enc." under your signature at the end of a letter means that you are enclosing additional documents with your letter. You might use this abbreviation on a cover letter if you are sending your resume, letters of recommendation or transcripts with the letter. This abbreviation helps the recipient know whether he has received all of the information that you tried to send. "P.S." under your signature of a letter means "post script." Use it when you have something else to say after you have finished your letter. It is usually used to indicate a short statement that you want to include in your letter that you forgot to mention. You might also use "P.T.O." to indicate that the letter is continued on the other side of the page. It means "Please turn over," and it is generally used in informal letters.
"PP" means "per procurationem" in Latin. Use it when you are writing or signing a letter on someone else's behalf. You may need to do this if the person cannot do so himself. "RSVP" is used to indicate that the recipient needs to reply to the letter or invitation. It is a French phrase, "répondez s'il vous plait," literally meaning "Respond if it pleases you."
Some titles that you would use in a letter before a person's name in the recipient's address or in the greeting are "Mr.," "Ms.," "Mrs.," "Dr." or "Prof." for professor. "Miss" is not an abbreviation, so do not place a period at the end of it. "Ms." is also not an abbreviation, but a period is placed at the end of it. If you are addressing a letter to more than one man, write "Messrs." "Drs." is the plural of "Dr." If you are writing to more than one married lady, write "Mmes" or "Mmes." This can be written with or without the period at the end.