Typography of multi-letter variables
In the International System of Quantities, i.e. the system of quantities that corresponds to the International System of Units (SI), as described in ISO 80000, the symbols for quantities are generally single letters from the Latin or Greek alphabet, sometimes with subscripts or other modifying signs. However, symbols for so-called characteristic numbers are written with two letters from the Latin alphabet, the initial of which is always capital. For example:


Reynolds number $Re$
$$Re=\frac{\rho vl}\eta=\frac{vl}\nu$$
Nusselt number $Nu$
$$Nu=\frac{Kl}\lambda$$
Prandtl number $Pr$
$$Pr=\frac{\eta c_p}\lambda=\frac\nu a$$


(Note that such two-letter symbols should be separated from other symbols if they occur as factors in a product.)

Quantity symbols are always written in italic type.

Various other widespread multi-letter symbols are not quantity symbols in accordance with the above-mentioned systems; they are actually just abbreviations. For example in toxicology, LD50 is used as abbreviation for “median lethal dose” (or “lethal dose, 50 %“). Such abbreviations are typeset normally, i.e. generally upright.

Also note that abbreviations for quantities shall not be used in equations.