[HarryT]

Thanks - that makes sense.

The other thing I always struggled with, when I was learning German, was what the "rules" were for using a "ß" instead of "ss". I used to think that "ß" was only used after a long vowel - eg "Straße" (street), but then I came across words like "Schloß" (castle) where the vowel is NOT long. Is there any "rule", or is it simply a matter of learning which words have a "ß" and which don't?