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Microsoft Compiled HTML Help

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Microsoft Compiled HTML Help
Filename extension
Internet media type
Developed byMicrosoft
Initial release1997
Latest release
Extended to.lit
Microsoft Compiled HTML Help
Operating systemMicrosoft Windows
PredecessorMicrosoft WinHelp
SuccessorMicrosoft Help 2
TypeHelp system

Microsoft Compiled HTML Help is a Microsoft proprietary online help format, consisting of a collection of HTML pages, an index and other navigation tools. The files are compressed and deployed in a binary format with the extension .CHM, for Compiled HTML. The format is often used for software documentation.

It was introduced as the successor to Microsoft WinHelp with the release of Windows 95 OSR 2.5[3] and consequently, Windows 98. Within the Windows NT family, the CHM file support is introduced in Windows NT 4.0[4][5][6] and is still supported in Windows 11.[7] Although the format was designed by Microsoft, it has been successfully reverse-engineered and is now supported in many document viewer applications.


Month Year Description
February 1996 Microsoft announces plans to stop development of WinHelp and start development on HTML Help.
August 1997 HTML Help 1.0 (HH 1.0) is released with Internet Explorer 4.
February 1998 HTML Help 1.1a ships with Windows 98.
January 2000 HTML Help 1.3 ships with Windows 2000.
July HTML Help 1.32 releases with Internet Explorer 5.5 and Windows Me.
October 2001 HTML Help 1.33 releases with Internet Explorer 6 and Windows XP.
March At the WritersUA (formerly WinWriters) conference, Microsoft announces plans for a new help platform, Help 2, which is also HTML based.
January 2003 Microsoft decides not to release Microsoft Help 2 as a general Help platform.

Microsoft has announced that they do not intend to add any new features to HTML Help.[8]

File format[edit]

Help is delivered as a binary file with the .chm extension. It contains a set of HTML files, a hyperlinked table of contents, and an index file. The file format has been reverse-engineered and documentation of it is freely available.[9][10]

The file starts with bytes "ITSF" (in ASCII), for "Info-Tech Storage Format", which is the internal name given by Microsoft to the generic storage file format used for CHM files.[11]

CHM files support the following features:

  • Data compression (using LZX)
  • Built-in search engine
  • Ability to merge multiple .chm help files
  • Extended character support, although it does not fully support Unicode.[12]

Use in Windows applications[edit]

The Microsoft Reader's .lit file format is a modification of the HTML Help CHM format. CHM files are sometimes used for e-books.[13]

Sumatra PDF supports viewing CHM documents since version 1.9.

Various applications, such as HTML Help Workshop and 7-Zip can decompile CHM files. The hh.exe utility on Windows and the extract_chmLib utility (a component of chmlib) on Linux can also decompile CHM files.

Microsoft's HTML Help Workshop and Compiler generate CHM files by instructions stored in a HTML Help project. The file name of such a project has the extension .HHP and the file is just a text with the INI file format.[14]

The Free Pascal project has a compiler (chmcmd) that can create CHM files in a multiplatform way.

Use in non-Windows applications[edit]

Read support:

Read/write support:

See also[edit]


  1. ^ Techtonik, Anatoly (11 April 2006). "application/vnd.ms-htmlhelp". Retrieved 7 March 2012.
  2. ^ "Microsoft HTML Help 1.4". Windows Dev Center. Microsoft. Retrieved 10 January 2017.
  3. ^ ^ since IE 4 is bundled with OSR 2.5, hence adding the CHM file support for the first time^
  4. ^ "Microsoft Security Bulletin MS02-055 - Critical". docs.microsoft.com. Retrieved 2020-10-29.
  5. ^ "Full Disclosure: HtmlHelp - .CHM File Heap Overflow". seclists.org. Retrieved 2020-10-29.
  6. ^ ^ The information available is scarce, but any VM installation of WinNT 4.0 appears to give .CHM file support out of the box. It seems like it wasn't introduced in in Windows NT 3.51 due to NT 3.51 being intentionally limited to Internet Explorer 16-bit versions^
  7. ^ "[SOLVED] Windows 10 CHM Help Files showing up blank. - Spiceworks". community.spiceworks.com. Retrieved 2020-10-29.
  8. ^ "Microsoft HTML Help Downloads". Microsoft. Retrieved 6 March 2012.
  9. ^ Wise, Paul; Wing, Jed (2005). "Unofficial (Preliminary) HTML Help Specification". Retrieved 15 August 2012.
  10. ^ Palade, Alexandru (2005). "Archive::Chm". Retrieved 22 July 2014.
  11. ^ "Virus Bulletin :: Chamber of horrors". www.virusbulletin.com. Retrieved 2022-01-05.
  12. ^ "INFO: Limited Unicode Support in HTML Help". Microsoft. 11 April 2001. Archived from the original on 13 March 2016. Retrieved 17 March 2012.
  13. ^ Salomon, David; Motta, Giovanni; Bryant, David (CON) (2009). Handbook of Data Compression (5th, illustrated ed.). Springer. ISBN 978-1-84882-902-2.
  14. ^ Wise, Paul; Wing, Jed (2005). "Unofficial (Preliminary) HTML Help Specification INI formats". Retrieved 1 February 2018.

External links[edit]