PC file extensions — backups: Which filename extension commonly indicates a backup copy of another file?

Introduction / Context:
Legacy DOS/Windows conventions use three-letter extensions to signal file types or purposes. Recognizing common extensions helps identify files quickly during troubleshooting and maintenance.

Given Data / Assumptions:

• Many programs create backup copies automatically when saving changes.
• Conventional backup extension is “.BAK”.
• Other extensions like .TXT, .COM, .BAS have distinct meanings.

Concept / Approach:

Users often see pairs like CONFIG.SYS and CONFIG.BAK. The .BAK file holds a prior version, enabling recovery if the new file is faulty. This convention persists across utilities and editors.

Step-by-Step Solution:

Verification / Alternative check:

Numerous editors and installers save previous configuration as *.BAK, confirming the convention.

Why Other Options Are Wrong:

• .TXT: Plain text file.
• .COM: Executable format under DOS.
• .BAS: BASIC program source.
• None of the above: Incorrect because .BAK is correct.

Common Pitfalls:

Assuming all backup files use .BAK; some tools use .OLD or timestamped names, but .BAK remains the most recognized generic backup extension.

Final Answer:

BAK