P.S. = 'Post Scriptum' - Latin for 'written afterwards'. Its meaning is 'Oops, I was just going to stuff this in the envelope and I remembered I hadn't put this bit in'. You should never add a P.S. to a formal letter, which is supposed to be carefully composed, but rewrite it. As for R.S.V.P., it stands for 'R�pondez s'il vous pla�t', which is French for 'please reply'. It is a formal request for a reply. If you see this on an invitation, not to answer would be bad manners.