Italic or upright letters for corners/points in geometry

Question

What is the recommended typeface for "corner points" (e.g. "the triangle ABC"), distances (e.g. "the side AB"), etc. in geometry? Should it be upright or italic? Some say all constants should be upright (even $e$ and $\pi$) and since "corner points" are constant they should therefore be upright. If a point on a line, say E between A and B, is variable, should it then be in italic, E, to indicate a variable? Any comments and thoughts about this are welcome. TIA.

Answer 1

$\triangle ABC$ ...

The question is not whether they are constant. The letter $A$ may be different points in different problems, and even in one problem the letter $A$ could represent any point in the plane. So, despite being "constant", these are variables, and thus they are set in italic nowadays.

Similarly, we write a line $y=ax+b$ where $a$ and $b$ are constants, and set all four variables $a,b,x,y$ in italic.