The letter ß (called an Eszett in German) is simply another way of writing ss and on its own is pronounced the same as ss.
A single s in German is typically pronounced like a z in English, whereas ss is always pronounced like the English s. Hence they use ss to make it clear which sound is to be used. It used to be that ß and ss were virtually interchangeable.
However, since the spelling reforms of recent years, the letter ß is no longer just a replacement for ss, it also affects the sound of the vowel preceding it. If a word is spelled with the Eszett, then the vowel is a long vowel; otherwise, it is a short vowel.
The letter combines the two forms of the letter s into one. It’s almost forgotten and not used anymore, but there is a second form of the letter s that was used in both English and German, called the “long s”.
It was used to replace the first instance of an s in a syllable or word. Hence the word sense would have been written ſense. You see the long s in older printed texts, and it is often confused with an f. It even shows up in the US Bill of Rights:
So if you combine the two — ſs — you get ß. This kind of joining of two letterforms into a new letterform is called a ligature. Normally a ligature (like those for fi or fl) is considered to be a new letterform, but not a new letter in its own right. However, in German, it did indeed evolve into a separate letter.
In fact people with ß or ss in their names get pretty annoyed if you get it wrong in their name. Strauß and Strauss or Groß and Gross are considered different spellings, whatever their pronunciation.
Confusingly, the German name Eszett literally means “s z”, that is, the letter s and letter z. This is because the phoneme ss was also often written sz in older German texts, and the original form of the ß letter did indeed combine the long s with a z. It first appeared sometime in the 15th century, and the ss form of the letter eventually won out while the Eszett name was kept. (It is also called a scharfes S, literally a “sharp S”.)
You do, however, sometimes still see versions of the ß that are true sz ligatures. One example, oddly enough, is Berlin street signs, which use their own unique font.
Street sign for Friedrichstraße in Berlin. Note the different shape of the ß letter that combines a long s and a z.
Another oddity of the letter ß is that it is actually only used in lower case. If a word is capitalized, then the ß is normally replaced by a capital SS. So groß is normally written in all caps as GROSS, which actually violates the spelling rule I mentioned above — technically groß and gross should be pronounced slightly differently (with a long or short vowel). On occasion you may see people use SZ instead of SS to make it clear that a ß was meant.
Hence there has been some effort to introduce a new capital ß. It looks like this: ẞ
However, while there is a Unicode space for it that was added in 2017, the capital version is not as yet included in many fonts. Thus it is not yet very commonly seen in German texts.
Another peculiarity is that while the ß is standard in High German as spoken in Germany and Austria, it is not in Switzerland or Liechtenstein.
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