Back in December, this appeared in the Ms. Mentor section of The Chronicle (sorry, it requires a subscription). Basically, one faculty member wrote in for advice about something petty; the instructor who preceded him or her (can't remember) in the classroom never erased the chalkboard, in spite of being left a note on the board asking that he do so, having the annoyance mentioned by a mutual colleague, and being emailed about it. Now, I was surprised by the letter, because I have no clue who uses the classrooms before me (I guess Ms. Mentor's correspondent must teach at a very small place), and even if I did, I would never write notes or emails or gossip about the subject. But since then I have been thinking about it, and wondering if there is an unspoken etiquette about chalkboard erasing. I notice that most of the time, the chalkboards in the rooms I use are not erased - they are full of scribbles. This is even true in the smart room. I always erase whatever is there when I enter - and also never fail to erase anything I write on the board before I leave. It just seems the appropriate thing to do, yet others do not extend the same courtesy.