Conventional sign designating a quantity or a unit of measurement.
Symbols for quantities are denoted by letters of the Latin or Greek alphabet printed in italic (sloping) type;
is an exception.
Symbols for units are denoted by letters of the Latin or Greek alphabet printed in roman (upright) type.
PAC, 1996, 68, 957
(Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996))
on page 995
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook