In a letter, as in any 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 the initials was added after the other material had already been written, often as an afterthought; as such, it typically occurs at the very end of the letter, usually below the signature. In some cases, a “P.P.S.” may appear below that, 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 a letter writer might add a post scriptum, known more commonly in English as a postscript. Most of the time, people include one when they remember something right before mailing a letter and don’t want to wait to include 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 so that the reader will be alerted to the fact that the old address is no longer valid, in case he or she misses the change in the return address section of the envelope.
Adding a Personal Touch
A postscript 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, or to offer a softer, more personalized ending. Postscripts are occasionally used in pre-printed letters to add a personal note to a form response.
One place where this technique is relatively rare is in formal business communications. Business correspondence is usually carefully composed without any unnecessary additions; information that is forgotten usually requires a complete re-write. Personal notes or caveats are typically out of place in these contexts.
As a Vehicle for External Comments
In some cases, a postscript 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 larger flow. They are often included in books for much the same reason — often to allow writers to thank people who have contributed to the work. Acknowledgments might be cumbersome in other areas of the book. Authors might also use the device to provide additional information, such as lists of resources readers might be interested in.
Cautions and Overuse Concerns
It is not uncommon to see P.S. written as “PS,” and both terms are generally viewed as stylistically acceptable. Writers should be careful about employing the postscript in excess or with frequency, however. It can be a highly useful and sometimes charming writing tool, but can become irritating if its use becomes a habit. Especially when composing formal correspondence, using one can devalue the seriousness of the letter.