I've recently read a book which used the sign $\therefore$ (for "therefore"). It was more or less clear from the context what was meant, but I looked it up among the AMS LaTeX symbols just to be sure. They also listed the similar $\because$ ("because") sign.
I might have seen this notation before, but very rarely if at all. Is this because I've studied in Germany and these symbols are used more often in other countries? Or are they very old and out of fashion? (The book I mentioned was rather new, though.) Or did I just read the wrong books?
Any information tidbits about where $\because$ and $\therefore$ come from and about their current or historical usage?
And, FWIW, I'd also be interested in whether there's some graphical or typographical clue hidden in these symbols (like the "vell" in $\lor$). How do you memorize which is which?