SS
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.
Origin
Today's ß was created around the 1900s. (See w:en:ß and w:de:ß for details on the so-called "Sulzbacher Form".) It has two origins, one in Blackletter and one in Roman type (also called Antiqua).
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.
Spelling
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 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.like ss is replaced with ß like in: floß
ß Media
Replacement street sign in Aachen, adapted to the 1996 spelling reform (old: Kongreßstraße, new: Kongressstraße)
Capitalization as SZ on a Bundeswehr crate (ABSCHUSZGERAET for the pre-reform spelling Abschußgerät 'launcher')
Use of ß in Polish, in 1599 Jakub Wujek Bible, in the word náßéy, which means our, and would be spelled naszej in modern orthography
Use of Middle High German letter “z” for modern “ß” in the beginning of the Nibelungenlied: "grozer" = "großer"
Use of the late medieval ligature ⟨ſz⟩ in Ulrich Füetrer's Buch der Abenteuer: "uſz" (modern German aus)