Drafting IEC publications
27 Mathematical formulae
27.1 Purpose or rationale
A mathematical formula uses symbols to express the relationship between quantities.
NOTE Notations such as
for numerical values are not mathematical formulae.
27.2 Title
Mathematical formulae do not have a title.
27.3 Numbering and subdivision
Mathematical formulae can be numbered in a document, for example for crossreferencing purposes. Arabic numbers in parentheses shall be used, starting with 1.
EXAMPLE 1

By default, the numbering shall be continuous and independent of the numbering of clauses, tables and figures. However, mathematical formulae may be numbered with a prefix and a number, where the prefix denotes the clause number, and the second number denotes the sequence of the formulae in the clause. The numbering system chosen shall be consistent within any given series. Subdivision of mathematical formulae [e.g. (2a), (2b), …] is not permitted.
When mathematical formulae in annexes are numbered, the numbering restarts and is preceded by the annex letter.
EXAMPLE 2 
27.4 Referencing
If a mathematical formula is numbered, it should be referred to in the text and its purpose should be made clear by its context, for example, with an introductory proposition.
Use, for example, the following forms for references to mathematical formulae:
 "see 10.1, Formula (3)";
 "see Clause A.2, Formula (A.5)".
27.5 Specific principles and rules
Mathematical formulae shall be expressed in mathematically correct form.
It is better to use mathematical formulae with quantities than mathematical formulae with numerical values.
However, the same symbol shall never be used within a document both for a quantity and for its corresponding numerical value. For example, use of the formula in Example 1 and of the formula in Example 2 in the same context would imply that 1 = 3,6 (which obviously is not true).
If, exceptionally, a formula between numerical values is used, the style shown in Example 2 shall be followed.
Variables shall be represented by italicized letter symbols. These shall be singleletter symbols except for exceptional cases such as characteristic numbers, which may be multiletter symbols.
The meanings of the symbols shall be explained in connection with the mathematical formulae, unless they appear in a symbols and abbreviated terms clause.
The same symbol shall not be used within a document both for a quantity and for its corresponding numerical value.
Descriptive terms or names of quantities shall not be arranged in the form of a mathematical formula. Names of quantities or multiletter abbreviated terms, for example presented in italics or with subscripts, shall not be used in the place of symbols.
The same symbol should not be used to represent different quantities within the same document. Subscripts can be useful to distinguish symbols for related concepts.
Unit symbols shall not be used within mathematical formulae.
Further examples are presented in Annex B.