ß
However, not every ss can be written as ß. German often puts two or more words together to make a longer word. If the new word has ss, it cannot be written as ß.  For example, Voßstraße is two words joined together (Voß and straße). It can also be written as Vossstraße but not as Vosßtraße because sstraße (or ßtraße) is not a word, and the ß is in the word Voss (Voß).