International standard ISO 2145 defines a typographic convention for the "numbering of divisions and subdivisions in written documents". It applies to any kind of document, including manuscripts, books, journal articles, and standards.
The ISO 2145 numbering scheme is defined by the following rules:
- Only Arabic numerals (1, 2, 3, …) are used.
- The main divisions are numbered continuously starting from 1.
- Each main division (first level) can be divided further into subdivisions (second level), which are equally continuously numbered. This can be continued for further levels of subdivision.
- A full stop is placed between numbers that designate subdivisions of different levels. No full stop is placed after the number that designates the final subdivision.
- A number 0 (zero) can be assigned to the first division of each level if it forms an introduction, preface, foreword, or the like.
A table of contents might look like:
Division and subdivision numbers are cited in written text as in:
- … in chapter 4 …
- … as lemma 3.4.27 shows …
- … the 3rd paragraph in 18.104.22.168 …
In spoken language, the full stops are omitted:
- "… in chapter four …"
- "… as lemma three four twenty-seven shows …"
- "… the third paragraph in two four one seven …"