ISO 31
International Standard ISO 31 (Quantities and units, International Organization for Standardization, 1992) was the most widely respected style guide for the use of physical quantities and units of measurement, and formulas involving them, in scientific and educational documents worldwide. In most countries, the notations used in mathematics and science textbooks at schools and universities follow closely the guidelines given by ISO 31. It is now superseded by the harmonized ISO/IEC 80000 standard.
Read more about ISO 31: Parts, Coined Words, Related National Standards