The Latin preposition de, from, takes the ablative, the concept behind of requires the genitive (yes I spelled it wrong before). The Latin for dam is claustrum, -i which is 2nd declension, neuter; the genitive would be claustri and the ablative would be claustro. How you would tell which it was from looking at the dam is beyond me; therefore we must take this in some other sense.

Thanks for the explanation, Faldage. I had got as far as trying to decline "de" into "dam" in Latin, and "der" into "dam" from German without any success.

The support group sounds an excellent idea, though I probably don't yet qualify. My sister in law gave my daughter one of those clocks which sing bird songs on the hour and was then mortified to discover there is a web based support group for people who have been given one!

Rod