I like there to be a difference between a variable “p” and a variable “P” (the former, for example, might be a vector, whereas the latter could conceivably be a projector onto this vector). Especially when doing numerical work, I found it to be extremely helpful to stick to established notation in the field also in the code intended to implement the algorithms written in this notation. In this sense, I find it sometimes sad that C++ doesn’t allow boldface/curly variable names :)