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Windows reserves CON as a device name, so it cannot be used as an ordinary file or folder name—even if you add an extension. The rule carries forward an MS-DOS convention: those short names referred to devices such as the console, printer, and null device.
Why can’t you name a file or folder “CON”?
CON is a reserved device name, not an ordinary word as far as Windows filename handling is concerned. The restriction applies to directory names as well as file names. Microsoft’s current Win32 naming guidance lists CON among the names to avoid.
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The historical reason is visible in Microsoft’s MS-DOS 3.20 User Reference (1986). It explains that MS-DOS treated certain device names specially: CON referred to keyboard input or terminal-console output, PRN to the printer, AUX to an auxiliary device, and NUL to a device used when a command needed a filename but no file should be created.
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Which names are reserved?
Microsoft’s current list includes these device names:
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CON,PRN,AUX, andNULCOM1throughCOM9LPT1throughLPT9COM¹,COM²,COM³,LPT¹,LPT², andLPT³—the superscript digits count as digits for this rule
The device-name idea is still reflected in Windows internals. Microsoft’s driver documentation describes DOS device names as symbolic links in the object manager and uses the serial port COM1 as an example. See Introduction to MS-DOS Device Names.
Why doesn’t adding an extension work?
The reserved name remains reserved when an extension follows it. Microsoft gives NUL.txt and NUL.tar.gz as examples treated as equivalent to NUL. The 1986 MS-DOS manual makes the same point with CON.XXX: it still refers to the console rather than an ordinary disk file.
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So adding a suffix such as .txt does not turn CON, NUL, or another listed device name into a normal filename. The rule concerns the reserved name itself; it does not mean that every longer name containing the letters “con” is prohibited.
Is this the same as Windows’ other filename restrictions?
No. The device-name restriction has a historical connection to DOS device references. Other naming guidance addresses how Windows’ shell and user interface handle names. Microsoft advises against ending a file or folder name with a space or period because the shell and user interface do not support those endings, even though an underlying filesystem may support them.
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Windows may also create an 8.3 short-name alias for a long filename, but this is a separate behavior: aliasing can be disabled, and it does not follow one universal rule across filesystems. These caveats do not change which device names are reserved.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should you do when choosing a Windows name?
- Avoid the reserved device names listed above for both files and folders.
- Do not rely on an extension to make a reserved name usable.
- Avoid ending a name with a space or period if it needs to work through the Windows shell and user interface.
- Do not assume every long filename has an 8.3 alias.
For the authoritative naming details, consult Microsoft’s Win32 filename guidance.
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