Hello,
In German orthography, double consonants mark the preceding vowel as
being short (if there isn’t just a mere co-incidence in a compound,
e. g. “Mausschwanz” (mouse tail)). As the “a” in “Straße” is long,
you write “ß”; as the “a” in “Gasse” is short, you write “ss”.
Cf. <https://grammis.ids-mannheim.de/rechtschreibung/6173>
and <https://grammis.ids-mannheim.de/rechtschreibung/6180>.
This is the new rule since the 1996 reform though. Originally, “ß” would
replace “ss” in all words where the two “s” belong to the same syllable,
including words with short vowels like “Fluß” (now “Fluss”) or “naß”
(now “nass”) as well as those with long vowels like “Fuß” (not changed)
or “Straße” (where the syllable boundary is before the “ß”, “Stra-ße”).
“Gasse” however is hyphenated “Gas-se”, so the two “s” never ligated to
a “ß” by either rule.
Curiously though, when I was in school in the 80s and 90s, we were
always taught the new rule even before the reform was in public
discussion, let alone in effect. The many short-vowel words with an “ß”
were called “exceptions”, despite having been perfectly regular if you
cited the rule that was actually in use at the time. I have never
understood how the old rule could fall into oblivion while it was still
used.
Kind regards,
Alexander
