Therefore

Three dots in a triangle, point up, and a proof suddenly has a conclusion. The symbol turns up in Johann Rahn's Teutsche Algebra of 1659, standing in for the Latin ergo — though whether Rahn or his tutor John Pell actually put it there is a question the record never settled. The two men worked closely on the book, and Pell's fingerprints are all over its notation.

Here is the part that surprises people: the convention you were taught is younger than the symbol by about two centuries. Rahn's own usage was not stable, and in the 1668 English edition the inverted triangle was used more often to mean "therefore" — the exact reverse of today's rule. Eighteenth-century authors went on using three dots in a triangle without agreeing which way up it belonged. Only in the nineteenth century did the split harden into what we now treat as obvious: point up for therefore, point down for because.

So if you find the two swapped in an old text, it isn't a typo. It predates the rule. One practical note for anyone setting type: a nearly identical glyph exists as a map symbol for a historic site, and Unicode flags the resemblance itself. Nothing but context tells them apart, because the shapes are effectively the same.