# Compiled Help (CHM) output

The transform type "htmlhelp" is used to create HTML Help control files. If the build runs on a system that has the HTML Help compiler installed, the control files will be compiled into a CHM file.

Once the preprocess and XHTML process is complete, most of the HTML Help processing is handled by the `dita.map.htmlhelp` target. This target creates several files:

- The HHP file is the control file for the HTML Help project.
- The HHC file contains the HTML Help table of contents.
- The HHK file contains the HTML Help index. This file is sorted based on the language of the map.

The `dita.htmlhelp.convertlang` is a post-processor for the content to ensure that it can be processed correctly by the compiler, and that the appropriate codepages and languages are used.

Finally, `compile.HTML.Help` attempts to detect the HTML Help compiler; if found, it compiles the full project into a single CHM file.

Parent topic: Generating XHTML with navigation
Related reference
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