The letter ß (also known as sharp S, German: Eszett or scharfes S) is a letter in the German alphabet. It is the only German letter that is not part of the basic Latin alphabet. The letter is pronounced [s] (like the "s" in "see") and is not used in any other language.
The letter came from the long s (ſ) and the normal z. Written fast by hand, they over time joined together to form a single glyph.
The ß is used only in German and never at the start of German words. The uppercase ß (ẞ) exists only for typesetting, such as in a dictionary. Instead of lowercase ß, one can also write ss. As no words start with double s or ß, no uppercase ß is necessary.
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).
The rules for German orthography have changed since 1996. Many common words that used to be written with ß are now written with ss. For example, Fluß (river) is now spelled Fluss. When the preceding vowel is short, as in Fluss, ss is used. However, the ß is used when the preceding vowel is long as in Straße.