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Windows does not let you name a file or folder exactly CON because it still recognizes certain names as references to devices, not ordinary items. The reserved list includes CON, PRN, AUX, and NUL, plus numbered COM and LPT names. Adding an extension such as .txt does not make one of these names available.
Why is CON reserved in Windows?
CON is a legacy device name for the console: keyboard input or terminal-console output. In the MS-DOS naming convention, these names referred to devices rather than ordinary files. Windows retains that device-name convention, so a name that looks like a filename can still be interpreted as a device reference.
Microsoft’s driver documentation describes DOS device names as symbolic links in the Windows object manager and uses the serial port name COM1 as an example. That shows the device-name concept remains part of Windows documentation; it does not mean that every reserved name maps to a physical device on every modern computer.
Which names are reserved?
Microsoft’s current Win32 filename guidance lists these reserved names for files and directories:
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CON,PRN,AUX, andNULCOM1throughCOM9LPT1throughLPT9COM¹,COM²,COM³,LPT¹,LPT², andLPT³
The superscript characters ¹, ², and ³ count as digits for these COM and LPT names. The restriction applies to the reserved device names themselves; it does not ban every longer name that happens to contain the letters “con.”
What the DOS-era names meant
Microsoft’s MS-DOS 3.20 User Reference, published in 1986, says that MS-DOS treated some device names specially. It describes AUX as an auxiliary device, CON as keyboard input or console output, PRN as the printer device, and NUL as a device reference used when a command required a filename but no file should be created.
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The manual also explains why an extension did not change the interpretation: CON.XXX still referred to the console, not a disk file. The current Windows rule has the same practical outcome for reserved names, even when the name is followed by an extension.
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Does adding an extension make the name usable?
No. Microsoft gives NUL.txt and NUL.tar.gz as examples equivalent to NUL. Appending an extension therefore does not turn a reserved device name into an ordinary filename. Use a different base name rather than trying variations such as CON.txt.
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How this differs from other Windows filename restrictions
Reserved device names are not the only naming concern, but nearby restrictions have different explanations. Microsoft advises against ending a file or directory name with a space or period because the Windows shell and user interface do not support those endings, even though an underlying filesystem may support them. That is a shell and UI limitation, not the DOS device-name rule.
Windows may also create an 8.3 short-name alias for a longer filename. Microsoft notes that this behavior can be disabled and varies by filesystem, so a long filename is not guaranteed to receive a short alias.
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What to do when a filename is rejected
- Check the base name. If it is one of the reserved names in the list above, choose a different base name.
- Do not rely on an extension. Names such as
NUL.txtremain reserved. - Check the ending separately. If the name ends in a space or period, remove that character; this is a separate Windows shell and UI compatibility issue.
For the current Windows rules, see Microsoft’s Naming Files, Paths, and Namespaces. For the historical examples, consult Microsoft’s MS-DOS 3.20 User Reference. Microsoft’s driver documentation explains MS-DOS device names in Windows’ object manager.
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