Why is “ss” equal to the German sharp-s character 'ß'?

Question

Coming from this question I\'m wondering why ä and ae are different(which makes sense) but ß and ss are treated as equal.

Answer 1

A few background facts:

• In Swiss German the eszet has been eliminated and replaced by ss in the 70s I think

• For uppercase conversion the official German replacement rule has always been and still is eszet->SS, even though an uppercase eszet has been defined for unicode (U+1E9E) a few years ago. I have never seen it in anywhere in the wild yet!

• No such changes and replacements have been made or have been necessary for the three umlaute äöü which have always had proper uppercase versions ÄÖÜ unless you don't have them. Replacing them by ae,oe,ue is only a workaround, though, hardly better than replacing a eszet by a beta or a 'B'..

So the different comparison results make at least some sense, although treatment, especially wrt sorting is not really reliably uniform in Germany between, say dictionaries or phone books, lists, indices etc..