I had one professor in university who called lowercase xi “squiggle” and wouldn’t make much effort to draw it properly. He would talk about, for example, “d squ

I had one professor in university who called lowercase xi “squiggle” and wouldn’t make much effort to draw it properly. He would talk about, for example, “d squiggle d t,” and so on. The advantage of a xi is that it looks nothing like an x so coordinate transformations (xi = f(x), tau = f(t)), are easier to keep track of.