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Mid-Michigan Computer Consultants, Inc.
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Bay City, Michigan

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We're writing these answers to the typical end-user. Other computer types will probably find things that they'd describe differently. If you are one of those people, please bear with us and see if you agree that the general idea is correct.


We frequently get a question like this:
What kind of a program can open a ".dat" file. I was sent a file like this and I can't read it.

The reason people ask this question is because they are clicking on a file and expecting Windows to open the file in some application. That usually works. When it dosn't, they don't know what to do.

To figure out what to do requires you to know a bit about how files are named and how Windows opens the files. Armed with some basic knowledge you can guess that a ".dat" file implies, but does not guarantee, that the file is some type of "data" file. If you can't open the file one way you can figure out another way.


File extensions are a useful file naming convention and they go back to the days well before the PC came along. An extension is a general method of classifying a file. For example, all programs may be ".EXE" files. All text documents may be ".TXT". Invoices from January may all have the extension ".JAN".

When the PC came along the file nameing rule was to use an 8 character file name followed by a period and a three character extension. This is frequently called an 8x3 file name. For example: thisfile.txt

Windows 95 introduced legitimate "long file names". Long names were found in other operating systems and there were methods of simulating long names even in DOS and Windows, but they were not officially recognized by DOS. Although Windows 95 allows the long names, it still stores files with an 8x3 name which is automatically carated from the long filename.

The file extension has also been expanded under Windows 95/98/NT. It is now possible to use, for example ".HTML" rather than just ".HTM".

There are few limitations on what a file extension can be. You can use any three alpha numeric characters in any combination. You can't use certain special characters (like the period itself).

There have always been some reserved file extensions on all systems. In the MS/DOS world, and now the Windows world, some of the most common reserved extensions are:
  • .BAT is a batch file with executable statements
  • .COM is a command or program
  • .EXE is ALSO a command or program
  • .GIF, .JPG, .TIF, etc. are certain types of image files.

Just because a file has a known extension does NOT mean that the file actually contains that type of file. You can rename ANY file to ANY name that you want. If you rename a .HTM file into a .JPG the file does not become a picture.


Windows allows you to "register" file extensions and the programs that you want to use to open that type of file. For example:
  • .JPG is an image file and Windows probably knows to open that file with an image viewer.
  • .PDF is an Adobe Acrobat reader file and should be opened with Acrobat
  • .HLP is a standard HELP file and will be opened with WINHLP32
  • .HLP is a standard HELP file and will be opened with WINHLP32
  • .MDB is a Microsoft Access file
  • .XLS is a Microsoft Excell spread sheet
  • .TXT, .DIC, .EXC are generally text files that will be opened with NOTEPAD.

When you install a new software package it will usually register it's own standard extensions and itself as the program to use to open it with.


If you get a file with an unrecognized extension, you're going to have to figure out what the file contains. Your best bet is to ask the person who gave you the file.

If you still can't figure it out, you can take a guess and try opening the file with some applications that MIGHT be able to figure it out. For example, one of our favorite image viewers is VuePrint from Hamrick Software. VuePrint doesn't care what the extension is; when you try to open a file of any type VuePrint will look at the contents and, if it IS an image file, VuePrint will display the image.

Another technique is to use a raw data viewing program to look at the file in a ASCII mode. Viewed in ASCII mode the file may look like total garbage, but it may also contain some readable information that will point you to the type of program that can handle the file.

There are a number of file viewing programs available as shareware. One of our favorites is an ancient program called "DR.COM". It was distributed by PC Magazine back in 1987. We use it because it is an absolute, bare bones, super simple program that will do no harm!

Many Windows 9x systems seem to include the "Quick View" program from Inso. Quick View trys to be smart and figure out what kind of file you're looking at, but you can usually convince it to just show you the contents in ASCII.


If nothing seems to work then you may need to get help. Consulting firms, like MMCC, can usually figure out what a file contains and can often extract the data and reformat it to a more useful format.

Send e-mail to MMCC.
Write to MMCC Technical Support at:
MMCC, Inc.
600 W. Midland
Bay City, MI 48708
(989) 686-8860
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