relating to the letter ß
The character ß evolved as a ligature of ‘long s and z’ (ſz) or ‘long s over round s’ (ſs). According to current official German orthography rules, the character ß is considered an individual letter.
The letter ß is unique in that it has no traditional upper case form. When using all-caps, current spelling rules require the replacement of ß with SS.
In Switzerland, ss usually replaces every ß. This is accepted by the current official German orthography rules.