The overall rule is that symbols representing physical quantities (or variables) are italic, but symbols representing units, or labels, are roman. Sometimes there may seem to be doubt as to whether a symbol represents a quantity or has some other meaning (such as a label): a good general rule is that quantities, or variables, can be given a value, but labels cannot. Vectors, tensors and matrices are usually denoted using a bold-face (heavy) font, but they should still be italic since they are still quantities.
Example: The mass of my pen m = 24 g = 0.024 kg. The electric field strength E has components Ex, Ey, and Ez. The Planck constant h = 6.626 068 76 (52) ×10–34 J s.
The above rule applies equally to letter symbols from both the Greek and the Latin alphabet, although authors often appear to resist putting Greek letters into italic.
Example: when the symbol µ is used to denote a physical quantity (such as mass or reduced mass) it should be italic, but when it is used in a unit such as the microgram, µg , or when it is used as the symbol for the muon, µ (see 5 below), it should be roman.
Numbers, and labels, are generally roman (upright), since they are not physical quantities.