You write N.B. (sometimes NB, with no dots) in front of a short note when you want the reader to stop and really look. It is a writer's tap on the shoulder: this bit matters. You see it on worksheets, in emails, in notices, and in contracts — much more in writing than in speech. When people do say it, they name the letters: “N B.” It feels a little formal and a little old-fashioned, like something a teacher or an office might put on a page; younger writers often just put Note: or Please note. Though actually, school papers and official documents still use it a lot. It is not the same job as P.S., which adds an afterthought after a letter is already finished. N.B. can sit anywhere you need a warning flag.