When I used to teach LaTeX classes, most of my students were math department secretaries, so I always made a point of explaining when someone would want \Pi versus \prod or \Sigma vs \sum.

Of course, being the callow youth that I was back then, I also tended to pronounce the names of the letters like a classicist so ψ is not pronounced "sigh" but rather "p-see", and similar ξ is not "zee" but "k-see." A childhood spent surrounded by people with Slavic surnames had my mouth well-trained to manage odd consonant clusters at the beginnings of words.

Everyone I know pronounces ξ as "double squiggly" and ς as "squiggly"

Back in physics grad school it surprised everyone else that I not only pronounced 'ξ' as "ksi" rather than "cascade" or "squiggle" but, more amazingly, could actually write it like ξ rather than a squiggle... I still don't really get why that's impressive.

The trick to writing ξ is to notice that it is a fast/sloppy cursive way of writing three parallel horizontal bars (Ξ).

Well, Cascade is the name of the particle, not the symbol... (and it would normally be upper-case, no?)

for non-physicists: https://en.wikipedia.org/wiki/Xi_baryon

Also, for this reason, surprised it's one of the more rarely used Greek symbols...

We had one prof who wrote these as ς with one, two, or even three squiggles below. I never figured out what letters they were supposed to be (zeta, sigma, xi?), but read them as something translating to "zig-zag".

ς and ζ look similar to their handwritten Roman equivalents.

ς is end sigma, the letter σ (S) when written at the end of the word.

ζ is zi (Z), it's not unusual to write a lower case Roman z with a similar tail. My mum was taught to do this, which confused me when I was learning to read.

ξ is xi (X).

Not 'fancy c' and 'fancy epsilon,' disrespectively? (which is a new word but one I think we should spread).

ζ and ξ are “squigma” while ς really is sigma.

yes... I hated it when professors used them on the board because I don't know how to write them other than making a squiggly...

The way I remember how to draw xi is horizontal line, epsilon, tail.

Agreed, definitely doesn't count. This nudges in \Theta which does surprise me a little

Yeah. Although -- I should note that the article did mention it as well, just toward the end. I was just a lazy reader on my first skim through and missed it.

I bet upper case theta is too close to lower case theta, and nobody wants to put their new variable so close to one with such a strong implicit meaning.