What is the CNT file format?
.cnt is a plain-text file that stores table-of-contents information for .hlp help files. .cnt files are created by Microsoft Help Workshop (HCW.EXE) and consumed by WINHELP.EXE to draw the Contents tab of a WinHelp help window. A .cnt file opens with colon-prefixed directives - :Base, :Title, and :Index - followed by a numbered tree of topic entries mapping captions to context IDs inside the paired .hlp file.
The .cnt and .hlp formats were replaced by more modern and secure standards. .cnt and .hlp text files were used by WinHelp to display help documents. The formats were improved over time and even came to support HTML formatting and scripts; those improvements also introduced security vulnerabilities. Ultimately, Microsoft decided to replace the .cnt and .hlp standard with HTML-based documents. The successor is .chm (Compiled HTML Help), which stores its table of contents internally.
Modern Windows (Vista and later) does not ship WinHelp by default, and WINHELP.EXE is no longer available from Microsoft for Windows 10 or Windows 11. It is possible to convert older help files to newer formats using migration tools distributed by Microsoft.
Security & safety
RISK: LOWA .cnt file is harmless plain text and cannot execute. The only real caution is the associated .hlp: Microsoft disabled WinHelp partly for security reasons (old .hlp files could run macros), so be wary of opening unknown legacy .hlp files even if you obtain the deprecated viewer. The .cnt itself carries no such risk.
Format details
in a nutshell- ColdFusion / generic counter file - Some web and database tools wrote plain-text .cnt files holding a single numeric counter value.
- Norton / application content index - A few legacy Windows apps used .cnt for internal content or settings lists; rare and app-specific.
Programs that open CNT files
Windows3 apps
Adobe RoboHelp
Paid Open the legacy WinHelp project that owns the .cnt to view/convert its contents; modern RoboHelp targets HTML/CHM output. 
Notepad
Open-source Open the .cnt for comfortable editing of the TOC tree with syntax highlighting and indentation. 
Microsoft Help Workshop (HCW.EXE)
Free Legacy authoring tool - opens the .cnt as part of a WinHelp project to rebuild the Contents tab. Hard to obtain today. Technical details
deep spec| Format type | Plain-text table-of-contents script; sidecar to a WinHelp .hlp file |
| Text encoding | ASCII / Windows ANSI code page (Windows-1252 typical) |
| File structure | Line-oriented: colon-directive header block (:Base, :Title, :Index) followed by an indented numbered tree of topic entries |
| File signature | No binary signature; a WinHelp 4.0 .cnt file begins with the text ":Base <name>.hlp" on the first line |
| Typical file size | 1 KB - 50 KB |
| Compression | None - stored as uncompressed plain text |
| MIME type | text/plain |
| Syntax variants | WinHelp 3.x used numbered heading lines only; WinHelp 4.0 (Windows 95) added colon-directive headers (:Base, :Index, :Title) |
| Consumed by | WINHELP.EXE - reads the .cnt file to build the Contents tab of the help window |
| Created by | Microsoft Help Workshop (HCW.EXE); also generated by RoboHelp and similar help-authoring tools |
| Associated operating systems | Windows 3.x, Windows 95/98/Me, Windows NT/2000/XP (WinHelp not included in Vista and later by default) |
| Role in help system | Defines the table of contents only; actual topic content and formatting reside in the paired binary .hlp file |
| Superseded by | .hhc (HTML Help table of contents inside .chm), then MS Help 2 (.h1k / .HxS), then web-based help |
| Developer | Microsoft |
| Released | 1990 (WinHelp, Windows 3.0 era); CNT format with WinHelp 4.0, Windows 95 |
| Latest version | WinHelp 4.0 :Base/:Index syntax (1995); format frozen - WinHelp deprecated after Windows XP |
CNT conversions
Community Q&A
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