The indexes are entered, stored and manipulated as plain text files. It is recommended that the files are given an extension ".idx".
It is recommended that only the printable ASCII characters are used. In particular accented characters are not guaranteed to sort correctly. (See Section 1.2.1 for how to do accents correctly.)
Headings and subheadings (ie. entries) occur one per line and can consist of any characters (although some characters have a special meaning). Entries must not contain two spaces in a row.
Subheadings are indented by two spaces relative to the previous level of headings. The number and depth of subheadings is unlimited. Because spaces are required ensure that your text editor does not convert multiple spaces into tabs.
Including the special characters ({|}*_\) when they're not part of a special construct will have strange results.
Example 1-1. Headings and subheadings
heading
subheading
another subheading
a sub-subheading
second heading