ß

The letter »ß« is unique to the German language. It is a called »scharfes s« (sharp s) or »sz«. If the letter is not available (or if it is capitalized) the »ß« is replaced by a »ss« (double s). In LaTeX use a »\ss«, with the babel package and the option german the short form is »"s«. In HTML you can use »&szlig;«.