"In the course of the Second Sound Shift in the 7th and 8th centuries, two different sounds emerged from Germanic /t/ and /tː/ - a fricative and an affricate - both of which were initially rendered with zz. Since Old High German, spellings such as sz for the fricative and tz for the affricate were used to differentiate between them."
"The sound written with ss, which goes back to an inherited Germanic /s/, differed from the sound written with sz; the ss was pronounced as a voiceless alveolo-palatal fricative [ɕ], whereas the sz was pronounced as a voiceless alveolar fricative [s]. Even when these two sounds merged, both spellings were retained. However, they were confused because no one knew anymore where an sz had originally been and where an ss had been."
"In the course of the Second Sound Shift in the 7th and 8th centuries, two different sounds emerged from Germanic /t/ and /tː/ - a fricative and an affricate - both of which were initially rendered with zz. Since Old High German, spellings such as sz for the fricative and tz for the affricate were used to differentiate between them."
"The sound written with ss, which goes back to an inherited Germanic /s/, differed from the sound written with sz; the ss was pronounced as a voiceless alveolo-palatal fricative [ɕ], whereas the sz was pronounced as a voiceless alveolar fricative [s]. Even when these two sounds merged, both spellings were retained. However, they were confused because no one knew anymore where an sz had originally been and where an ss had been."
https://de.m.wikipedia.org/wiki/%C3%9F