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

If you look at the Ä page, you'll see that not always Ä is a replacement for Æ (or ae), and it is still used in various languages.

The letter ß instead:

While the letter "ß" has been used in other languages, it is now only used in German. However, it is not used in Switzerland, Liechtenstein or Namibia.[1] German speakers in Germany, Austria, Belgium,[2] Denmark,[3] Luxembourg[4] and South Tyrol, Italy[5] follow the standard rules for ß.

So the ß is used in a single language, with a single rule (ß == ss), while the Ä is used in multiple languages with multiple rules.

Note that, considering that case folding is:

Case folding is primarily used for caseless comparison of text, such as identifiers in a computer program, rather than actual text transformation

The official Unicode 7.0 Case Folding Properties tells us that

00DF; F; 0073 0073; # LATIN SMALL LETTER SHARP S

where 00DF is ß and 0073 is s, so ß can be considered, for caseless comparison, as ss.

Some background info for you. Taken from here.

Windows Alt Codes

In Windows, combinations of the ALT key plus a numeric code can be used to type a non-English character (accented letter or punctuation symbol) in any Windows application. More detailed instructions about typing accents with ALT keys are available. Additional options for entering accents in Windows are also listed in the Accents section of this Web site.

Note: The letters ü, ö, ä and ß can be replaced by "ue", "oe", "ae" or "ss" respectively.

German ALT Codes

Sym Windows ALT CodeÄ   ALT+0196ä   ALT+0228Ö   ALT+0214ö   ALT+0246Ü   ALT+0220ü   ALT+0252ß   ALT+0223€   ALT+0128

Taken from here.

In the German alphabet, the letter ß, called "Eszett" (IPA: [ɛsˈtsɛt]) or "scharfes S", in English "sharp S", is a consonant that evolved as a ligature of "long s and z" (ſz) and "long s over round s" (ſs). When speaking it is pronounced [s] (see IPA). Since the German orthography reform of 1996, it is used only after long vowels and diphthongs, while ss is written after short vowels. The name eszett comes from the two letters S and Z as they are pronounced in German. It is also called scharfes S (IPA: [ˈʃaɐ̯.fəs ˈʔɛs, ˈʃaː.fəs ˈʔɛs] in German, meaning "sharp S". Its Unicode encoding is U+00DF.

Just wait half a century.

This year, after over a century of dispute, German added officially the ẞ as a valid uppercase replacement for the lowercase version ß. It will take some time before people get used to the new uppercase form ẞ, but as soon as the capital version ẞ will dominate, there will be no reason to continue this evil

String.Equals("Mr. Meißner", "Mr. Meissner", StringComparison.CurrentCulture) == true;

hack.