Hans Aberg asked: > I find this interesting: Do they state a reason for this recommendation, > so that there is a general principle behind it, like the idea with > typesetting constant names upright? You may call it general principle. The following conventions are often employed by physicists: Variables: math italics Vectors: bold math italics Operators: upright Vector Operators: bold upright Tensors: sans serif Constants are usually also in math italics (generally for physical constants like speed of light $c$), only \emph{numbers} like e, i, and pi occur upright depending on the publishers style. Differentials are also often upright. If you want to see a journal which follows all those conventions, look at Il nouvo cimento. --J"org Knappen