Or double flats or double sharps, which lower or raise the note one whole step, respectively.
(Rat feels his brain have a hiccup as he tries to grasp this information.)Ummm....if you're going to change a note by a whole step, why not simply, y'know, write the note to be one step higher or lower in the first place? It's been more than two decades since I played in my high school's band, but as I recall, sheet-music staffs do allow for that.
![Huh ???](http://fqa.digibase.ca/Smileys/default/huh.gif)