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Q: Why do some file types use both three and four letter extensions?

A: Most file types only have one file extension. However, in some cases, three and four letter extensions can be used for the exact same type of file. For example, the only difference between a .TIF and .TIFF file is the file extension. The files are exactly the same, except for the fact one uses a three letter file extension, while the other uses a four letter extension. If you were to change a .tiff extension to .tif, the file would still open the same way.

So why are there two different extensions for the same file type?

In early versions of Windows, all files required a three letter file extension. This meant that some extensions needed to be abbreviated to fit the three letter format. For example, TIFF (Tagged Image File Format) files used the shortened ".tif" extension to fit the format required by Windows.

While the Macintosh platform was slow to adopt file extensions, it also never limited file extensions to three letters. Therefore, several years ago, files with four letter extensions began appearing on the Macintosh platform. Fortunately, both Windows and the Mac OS now support longer file extensions, which enables them to recognize file types that use both three and four letter extensions. For example, some programs (particularly on the Mac), save TIFF files with a ".tiff" extension, because it is slightly more descriptive than the ".tif" extension.

However, to avoid confusion, most programs still stick with the ".tif" extension for TIFF files. Adobe Photoshop, for example, saves all TIFF files with a ".tif" extension on both Mac and Windows. The same holds true for other file types that use both three and four letter file extensions.

What other file types use both three and four letter extensions?

TIFF files are not the only type of file that share more than one extension. Here are a couple of other common examples of file types that use both three and four letter extensions:

While having multiple file extensions can seem rather ambiguous, it is usually pretty easy to determine the file type either way. Still, if you have a choice to save a file with a three or four letter extension, using three letters is a more standard convention. Therefore, with the exception of .HTML files (which commonly use the ".html" extension), it is best to use three letter file extensions whenever possible.

Updated: June 14, 2011