Somehow, somewhere, someone decided that texture coordinates would be represented using the variables "u" and "v". I don't know if anyone has noticed, but these two characters are remarkably similar, especially when handwritten. Worse yet, when converting from barycentric coordinates to texture space, these wonderful variables often inherit a "t" and become "tu" and "tv". In my handwriting, "t" looks like a plus sign. In the end, all derivations involving textures end up as a scribbled morass. I've been forced to use serifs when I write.
It's very frustrating.
While I am disgruntled at the 3D community for perpetuating this madness, I've always suspected it is a long running joke. In my digital controls classes, professors would often choose "s" and "t" as variables in transfer functions. "s" looks like a five, and I've already expressed my opinion about "t". Honestly, if you're running out of symbols for variables, consider using Wingdings. Or go Cyrillic.