I was in French immersion as a kid.

I first saw it when I was in Germany. I think it helps differentiate it from a "1," which tends to have a bit more exaggerated top slash in some European scripts.

and some of my aunts (her daughters)
They were all born here, but their grandparents were not. (German/Irish).
So I remember seeing it when growing up.

Beautiful Copperplate writing, btw.

for serif fonts. Interestingly, some sans serif fonts still put the serifs on the "1."

to distinguish from one, which usually looks like an open triangle

If you put the little tag at the top of a one, you have to follow up with a short horizontal line at the base. 1

I do a lot of genealogy and there is a lot of confusion in old records about 1s and 7s, Os (letter) and 0s (number), etal. I really, REALLY wish the US had the convention of listing dates as day month year. For my records, I use numerical day, abbreviated month and four digit year (I.E.: 22 Oct 2013) so there is no punctuation or other symbols to confuse the issue. My MIL in her records used various ways, which gets really annoying when you realize that every date for that branch of the family has the month and day transposed.

To make it worse, many now don't know how to read cursive, especially antiques versions. I spent most of yesterday hunting records on an ancestor whose middle initial was Y - but that initial was transcribed as G, T and A. Even worse is the ancestor whose last name is Tucker - the first initial of which has been (more often than you'd expect) transcribed as F, and sometimes as L, I, or P. "Lucker" or "Pucker" I can see, but "Fucker" as a surname?

We all need a way to tell if that is a 1 or a 7 or a 2.
It's a chemist's thing. Also a programmer's way of telling 0s from Os.
My problem nowadays is I can't tell my 5s from my 3s.

We use a slash through the 7 and 0, to make it easier for data entry people.

I learned in Europe.

It distinguished it from the ONE, which had a little "thingy" on the top, much like the seven, only shorter.

is called a serif. It's one of those words that probably only a coin or stamp collector would know

Web design etc...

a triggernometry warning...

Older Germans also make 1's that look like upside down V's .

That's how I was taught in Italy. Curiously, when I ended up in the middle of NC in high school, I was told not to cross my 7 because to do so was a satanic symbol (I'm not kidding). That's rural NC for ya.

ever since i got out of the French Foreign Legion

I picked up the habit when I was there

The way most Europeans write 1 is with a long hook on the left that makes it resemble a 7.

My parents, both born and raised in Europe, always put the line in the 7. Although I was born and raised in the U.S., I learned do it that way too, but to be honest, it's just an affectation on my part.

And I always thought it was cool to do it when I was in school, same as drawing a line through a zero, so as not to confuse it with an O. I still do that one at work.

Always does it. I just do it out of habit, have been since I was a kid.

I knew who wrote it like that (in Western Mass) went to Catholic school.

European 1's don't have the little serif at the bottom, so people dash the 7 to distinguish it from 1. The danger is that can then look like 4.

I have a friend who acquired the habit in college while studying French. I cross my Zs as well.

I do it and I'm an accountant.

Sometimes when you're writing numbers quickly, at least for me, my 1's and 7's kind of look the same.

I have to admit, you have me pegged.

I don't see how one can confuse that with any other letter.

the number 1 looks more like the greek letter 'lambda'. Putting a horizontal line through the number 7 clearly distinguishes it from 1.

But I do put a line through Z, I guess so it doesn't look like a 2.