Many mathematical publications assign numbers to displayed equations as a handy means of cross-referencing. In single-chapter and most multiple-chapter documents, in each chapter (including appendices) the equations begin with number 1. The entire number is enclosed in parentheses and set flush right opposite the displayed equation, which should not extend to within an em space of it (the equation itself is centered). See Sample. References to these numbers in the text are also enclosed in parentheses [e.g., see eq. (2)].
When an equation takes up several lines, or when a single number refers to a series of equations displayed together, the number is set on the last line of the display. See Sample.
Code-Specific Information
In some multiple-chapter/section codes and standards such as PTC and V&V documents, equations take the number of the first-level head in which they appear followed by a hyphen and a number reflecting their chronological order in that paragraph (see samples 1, 2, and 3).
