The German alphabet consists of the twenty-six letters of the ISO basic Latin alphabet plus four special characters: ä, ö, ü, and ß.
The three umlauted vowels (ä, ö, ü) can be written as "ae", "oe", and "ue" respectively when the umlauts are not available. The letter ß (called Eszett or scharfes S) can be written as "ss".
In the German phonetic alphabet, these special characters have distinct pronunciations that are important for proper German speech.