In a written communication, “P.S.” stands for the Latin phrase post scriptum, which means “after writing.” It is meant to reflect the fact that the text marked with “P.S.” was added after the other material had already been written, as an afterthought. You may also see a “P.P.S.” below a “P.S.,” and potentially one could create a cascade of “P.P.P.S.'s” and “P.P.P.P.S.'s,” although this is generally viewed as bad form.

There are a number of reasons to add a post scriptum, known as a postscript in English. Most commonly, people include a postscript when they remember something right before mailing a letter, and wish to addend it rather than including it in a future communication. For example, someone might remember that his or her address has changed, and add “P.S. My new address is...” so that the reader will be alerted to the fact that the old address is no longer valid, in case he or she misses the change in the return address section of the envelope.

A P.S. may also be used to add a comment to a written document, as in the case of a writer who wants to expand upon something in a letter without interfering with the flow of the letter. Postscripts are often included in books for much the same reason, and to allow writers to thank people who have contributed to the work. Acknowledgments might be cumbersome in other areas of the book, but they are entirely appropriate in the postscript. Authors also use postscripts to provide additional information, such as lists of resources readers might be interested in.

A P.S. can also be included to add a more informal touch to a formal communication, as in “P.S. George and the kids say hello,” reminding the reader of a personal connection to the writer. Formal letters may also use the postscript as a tool to provide more information about the context of the letter, although the postscript is relatively rare in formal business communications, since such communications are supposed to be carefully composed, without any unnecessary addendums. Additionally, postscripts are occasionally used in pre-printed letters to add a personal note to a form response.

You may see P.S. written as “PS,” and both terms are generally viewed as stylistically acceptable. Writers should be careful about employing the postscript, as it can be a highly useful and sometimes charming writing tool, but it can become irritating if it becomes a habit. Especially when composing formal correspondence, using a postscript can devalue the seriousness of the letter, so people should think carefully before adding a “P.S.”