File extensions are the suffix at the end of a filename (like .txt, .jpg, .docx) that indicate the file type and what software can open it. When you "rename the base name," you are changing the part of the filename that comes before the extension. Preserving the extension means ensuring this suffix remains unchanged during the renaming process. This differs from changing the extension itself, which alters how the operating system and applications interpret the file.

This practice is essential in common file management scenarios. For example, when updating a photo filename from vacation_photo1.jpg to family_beach_trip.jpg, you keep the .jpg extension so image viewers recognize it. Similarly, a programmer might rename a code file from module_old.py to module_new.py, preserving .py to ensure it runs correctly. Most operating systems (Windows Explorer, macOS Finder) and file managers handle this automatically if you only edit the name before the last dot.

Keeping the extension intact prevents issues like making a file unopenable because the software doesn't recognize it, or causing system errors. Accidental deletion of the extension during manual renaming is a common risk. Always ensure the dot and the characters immediately following it remain when you change the base name. Modern file managers and command-line tools are generally designed to avoid altering the extension unless explicitly instructed.