File: groff.info,  Node: Indexing,  Next: Document Formats,  Prev: Table of Contents,  Up: Common Features

3.2.8 Indexing
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An index is similar to a table of contents, in that entry labels and
locations must be collected, but poses a greater challenge because it
needs to be sorted before it is output.  Here, processing the document
in multiple passes is inescapable, and tools like the 'makeindex'
program are necessary.