IEC 60027 (formerly IEC 27) is a technical international standard for letter symbols published by the International Electrotechnical Commission (IEC), comprising the following parts:
A closely related international standard on quantities and units is ISO 31. The ISO 31 and IEC 60027 Standards are being revised by the two standardization organizations in collaboration. The revised harmonized standard is known as ISO/IEC 80000, Quantities and units. It supersedes both ISO 31 and part of IEC 60027.
IEC 60027-2 Amendment 2, as published in January 1999, was the first international standard defining the binary prefixes, as proposed by International Electrotechnical Commission (IEC) since 1996 (kibi- (Ki), mebi- (Mi), gibi- (Gi) and tebi- (Ti)) but extended them up to pebi (Pi) and exbi- (Ei). This didn't change in the second edition of the standard, published in 2000, but the third edition in 2005 finally added prefixes zebi- (Zi) and yobi- (Yi). The harmonized ISO/IEC IEC 80000-13:2008 standard supersedes subclauses 3.8 and 3.9 of IEC 60027-2:2005. The only significant change is the addition of explicit definitions for some quantities.