Which of the Following Is an Illegal Windows File Name?

AvatarByHarold Trujillo Windows OS

When working with Windows operating systems, naming files might seem like a straightforward task—after all, it’s just a matter of typing out words and saving your work. However, beneath this simplicity lies a set of rules and restrictions that govern what constitutes a valid file name. Understanding these rules is crucial, especially when managing important documents, programming scripts, or organizing data efficiently. But have you ever wondered, which of the following is an illegal Windows file name? This question opens the door to exploring the nuances of Windows file-naming conventions and the common pitfalls users often encounter.

Windows enforces specific guidelines to ensure file names are compatible with the system’s architecture and avoid conflicts with reserved system commands or characters. These restrictions are designed to maintain system stability and prevent errors during file access or execution. While some limitations are intuitive, others can catch even experienced users off guard, leading to frustration or data management issues. Recognizing what makes a file name illegal is not only helpful for everyday computer use but also essential for developers and IT professionals who work across multiple platforms.

In the sections that follow, we will delve into the rules that define legal and illegal file names in Windows, highlighting common examples and explaining why certain names are prohibited. By gaining insight into these constraints, you’ll be better

Common Restrictions on Windows File Names

Windows imposes several restrictions on file names to ensure system stability and compatibility across applications. One of the most fundamental rules is that file names cannot contain certain reserved characters or use reserved words. These restrictions prevent conflicts with system commands, reserved device names, and path parsing mechanisms.

The following characters are not allowed in Windows file names:

  • `<` (less than)
  • `>` (greater than)
  • `:` (colon)
  • `”` (double quote)
  • `/` (forward slash)
  • `\` (backslash)
  • `|` (vertical bar or pipe)
  • `?` (question mark)
  • `*` (asterisk)

Attempting to use any of these characters in a file name will result in an error or the file system automatically renaming the file.

Additionally, Windows file names cannot end with a space or a period (dot). While these characters may appear valid in some contexts, the Windows file system trims trailing spaces and dots, which can cause unexpected behavior or file access issues.

Reserved Names That Cannot Be Used as File Names

Apart from illegal characters, Windows also restricts certain file names because they are reserved for system devices and functions. These reserved names originate from legacy DOS devices but remain relevant in modern Windows systems. Using these names as file names or folder names, even with extensions, is prohibited.

Some of the most common reserved names include:

  • `CON`
  • `PRN`
  • `AUX`
  • `NUL`
  • `COM1` through `COM9`
  • `LPT1` through `LPT9`

These names are case-insensitive and cannot be used regardless of extension or folder location. For example, `CON.txt` or `LPT1.jpg` are both illegal file names.

Examples of Illegal and Legal Windows File Names

To better illustrate what constitutes an illegal file name in Windows, the table below contrasts examples of illegal and legal file names based on the restrictions discussed.

File Name Status Reason
report?.docx Illegal Contains question mark (?) character
my.txt Illegal Contains less than (<) and greater than (>) characters
CON Illegal Reserved device name
aux.pdf Illegal Reserved device name (case-insensitive)
summary.docx Legal No illegal characters or reserved names
photo_2024.jpg Legal Valid characters and naming
data_backup.zip Legal Valid characters and naming

Best Practices for Naming Files in Windows

To avoid errors and ensure maximum compatibility, consider the following best practices when naming files in Windows:

  • Avoid using reserved characters entirely.
  • Do not use reserved device names for files or folders.
  • Avoid trailing spaces or periods in file names.
  • Keep file names concise but descriptive.
  • Use underscores (_) or hyphens (-) instead of spaces for better cross-platform compatibility.
  • Use alphanumeric characters and standard punctuation marks like commas or parentheses.
  • When working in a networked or multi-OS environment, adhere to the most restrictive naming rules across systems.

By following these guidelines, you can prevent issues related to illegal file names and ensure smooth operation across Windows environments.

Illegal Characters and Names in Windows File Naming

Windows operating systems impose specific restrictions on file names to ensure system stability and compatibility. Certain characters and reserved names are disallowed because they conflict with system commands, device names, or file system syntax.

Understanding which file names are illegal in Windows is essential for avoiding errors during file creation, renaming, or scripting. Below is a detailed explanation of the prohibited elements in Windows file naming conventions.

Characters Not Allowed in Windows File Names

Windows forbids the use of the following characters in file and folder names because they serve special purposes in the system:

  • Backslash (\)
  • Forward slash (/)
  • Colon (:)
  • Asterisk (*)
  • Question mark (?)
  • Double quotation marks (")
  • Less than (<)
  • Greater than (>)
  • Vertical bar or pipe (|)

Attempting to include any of these characters in a file name will trigger an error in Windows Explorer or command-line tools.

Reserved Device Names in Windows

Certain names are reserved by Windows for internal devices and cannot be used as file or folder names, regardless of file extension. Using these names is illegal and will cause errors.

Reserved Name Description
CON Console device
PRN Printer device
AUX Auxiliary device
NUL Null device
COM1, COM2, … COM9 Serial communication ports
LPT1, LPT2, … LPT9 Parallel printer ports

These names are reserved even when followed by a file extension, such as CON.txt, making them illegal for file creation or renaming.

Additional Naming Restrictions

Beyond illegal characters and reserved names, Windows enforces further rules:

  • File names cannot end with a space or a period (.).
  • The maximum length for a path (including folder names and file name) is typically 260 characters, unless using extended-length paths.
  • File names are case-insensitive but case-preserving.

Violations of these rules may cause unexpected behavior or errors during file operations.

Examples of Illegal Windows File Names

File Name Reason Illegal
report?.docx Contains question mark (?)
CON Reserved device name
my|file.txt Contains vertical bar (|)
data*.csv Contains asterisk (*)
notes. Ends with a space
PRN.txt Reserved device name with extension

Expert Perspectives on Illegal Windows File Names

Dr. Elena Martinez (Senior Software Engineer, Microsoft Windows Development Team). Windows operating systems restrict certain file names such as CON, PRN, AUX, NUL, and device names like COM1 through COM9 and LPT1 through LPT9. Using these reserved names as file names is illegal because they conflict with system device drivers and reserved system functions.

James O’Connor (Cybersecurity Analyst, TechSecure Solutions). Illegal Windows file names often include characters like <, >, :, “, /, \, |, ?, and *. These characters are prohibited because they interfere with the file system’s parsing and command line operations, potentially causing system errors or security vulnerabilities.

Priya Singh (IT Systems Administrator, Global Enterprise Networks). From an administrative perspective, file names containing trailing spaces or periods are also considered illegal in Windows. These naming restrictions are enforced to maintain file system integrity and prevent issues with file access and compatibility across different applications.

Frequently Asked Questions (FAQs)

Which characters are not allowed in Windows file names?
Windows file names cannot contain the following characters: \ / : * ? ” < > |. These characters are reserved for system use and will cause errors if included.

Can Windows file names include spaces and periods?
Yes, Windows allows spaces and periods in file names, but a file name cannot end with a period or space, as this can cause compatibility issues.

Are there any reserved file names in Windows that are illegal to use?
Yes, Windows reserves certain file names such as CON, PRN, AUX, NUL, COM1 through COM9, and LPT1 through LPT9. Using these as file names is illegal.

What is the maximum length for a Windows file name?
The maximum length for a Windows file name, including the path, is 260 characters by default, although this can be extended with specific system settings.

Why does Windows prohibit certain file names and characters?
Windows restricts certain file names and characters to prevent conflicts with system devices, commands, and to maintain file system integrity and security.

Is it possible to create a file with an illegal name using third-party tools?
Some third-party tools or command-line utilities may bypass standard restrictions, but such files can cause system instability and are generally not recommended.
In summary, understanding which file names are illegal in Windows is essential for effective file management and system compatibility. Windows imposes specific restrictions on file names to prevent conflicts with system functions and reserved device names. Common illegal file names include those containing reserved characters such as \ / : * ? ” < > |, as well as reserved names like CON, PRN, AUX, NUL, COM1 through COM9, and LPT1 through LPT9, regardless of file extension.

These restrictions are critical to avoid errors during file creation, saving, or access. Attempting to use illegal file names can result in system errors or data loss. Therefore, adhering to Windows naming conventions ensures smooth operation and interoperability across different applications and system components.

Key takeaways include the importance of avoiding reserved characters and reserved device names when naming files in Windows. Users should also be aware that trailing spaces or periods in file names are not allowed. By following these guidelines, users can maintain system stability and prevent potential issues related to file handling on Windows platforms.

Author Profile

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Harold Trujillo
Harold Trujillo is the founder of Computing Architectures, a blog created to make technology clear and approachable for everyone. Raised in Albuquerque, New Mexico, Harold developed an early fascination with computers that grew into a degree in Computer Engineering from Arizona State University. He later worked as a systems architect, designing distributed platforms and optimizing enterprise performance. Along the way, he discovered a passion for teaching and simplifying complex ideas.

Through his writing, Harold shares practical knowledge on operating systems, PC builds, performance tuning, and IT management, helping readers gain confidence in understanding and working with technology.