I use the whole alphabet and more:
- a, b, c, d for constants
- e is Euler‘s constant
- f, g, h for functions
- i, j, k, l for indices but can be used for natural or whole numbers
- m, n for natural numbers but can be used as indices
- o, p, q, r, unreserved although I sometimes use p and q in a rational equation like: p / q
- s, t function parameters
- u, v, w vectors
- x, y, z variables
Greek letters for angles or literally anything else.
As uppercase, yes