How do I know to use ß or ss??
Like do I say wäßrig or wässrig, Litfaßsäule or Litfasssäule? Is it just up to choice?
Also note that there was a major reform in the 90s (’Rechtschreibreform’) that also changed the rules for ss/ß (e. g. ‘daß’ became ‘dass’). There are still some people who stick to the old rules, so don’t be confused if you see different versions of the same word.
"Die Litfaßsäule" is a special case, as the inventor's name was "Litfaß" and names do not really care about rules but argue with history. The "a" is spoken short here.