Indices in Info format are generally written as a menu (see Indices), but with an additional directive at the beginning marking this as an index node:
<printindex> = ^@^H[index^@^H] * Menu: <index entry>*
The <index entry> items are similar to normal menu entries, but the free-format description is replaced by the line number of where the entries occurs in the text:
<index entry> = * <entry text>: <entry node>. <lparen>line <lineno><rparen>
The <entry text> is the index term. The <lineno> is an unsigned integer, given relative to the start of the <entry node>. There may be arbitrary whitespace after the colon and period, as usual in menus. Here is an example:
^@^H[index^@^H] * Menu: * thunder: Weather Phenomena. (line 5)
This means that an index entry for ‘thunder’ appears at line 5 of the node ‘Weather Phenomena’.