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File Extensions

Introduction to file names
A filename is divided into two parts - the name and the extension.  On some systems, filenames must comply with what is known as the 8.3 naming convention - all this means is that there can be no more than eight letters in the filename, followed by a dot, and then three letters after the dot.  Most computers now will deal with longer filenames, so the days when files had to be given names like jslet3sl.doc or names truncated as in tchaikov.doc are thankfully over. 

However there are still occasions when 8.3 filenames are used.  Particularly if you are transferring information from one computer to another (and you don't know what system is being used) or one medium to another (i.e. hard disc to CD-ROM), it is a good idea to minimize risk by putting information which differentiates one file from another in the first eight letters, just in case.   For example, let's say you had three files about Swan Lake on your computer -

Swan Lake history.doc
Swan Lake background.doc
Swan Lake music.doc 

This is all fine on your computer.  You save your work to floppy disc, and then take it to a friends house, who has an older computer which cannot deal with filenames not in 8.3 format.  This is what you'll see -


Better then, either to create a folder called swanlake (which is thankfully only eight letters long) and then create files within that folder called history.doc, bkground.doc, music.doc like so:

c:\my documents\ballet\swanlake\history.doc
c:\my documents\ballet\swanlake\background.doc
c:\my documents\ballet\swanlake\music.doc

or use an prefix like SL for things to do with Swan Lake, so you get SLhistory, SLbackground.doc, SLmusic.doc.  Like this, you can still have a file with characters (if you must) but the most relevant detail will be within the first eight characters. 

Reserved Characters
Reserved characters are characters which cannot be used in a filename, because they have been "reserved" by the system to denote other things.  These characters are:

The chevron <
The asterisk *
The question mark ?
The forward slash /
The backslash \
Quotation marks "
Pipe symbol

It is easy to see why the backslash (\) is a reserved character - it denotes, as we have seen, a directory or folder.  Thus, a file cannot be called exercises\enchainements.doc  because the slash would indicate that there was a folder called "exercises" and in that folder there is a document called "enchainements.doc". 

Case Sensitivity and Spaces
Many systems allow uppercase letters in filenames, and spaces, and 255 characters in a filename. Wonderful news - but again, prepare for the worst.  Case-sensitivity means that your computer will differentiate between Letter.doc and letter.doc - in other words, it is sensitive to the fact that one L is upper case, and the other l is lower case. 

Calling one file Letter.doc and another letter.doc is not a good idea - will you remember which one contains what? Furthermore, just because your  computer knows the difference, does not guarantee that someone else's will.

Likewise, not all systems like spaces in filenames, so you might like to adopt a policy for filenaming which will not crumble under pressure - particularly if you are publishing files on the internet, or sending files for publication.  A fairly catch-all, failsafe policy is no spaces, all lower case, i.e.

John Smith letter 1998.doc



File Extensions
The three letters after the dot are known as the file extension, and tell you (and the computer) what sort of file it is.  You are probably familiar with the .doc extension, used for Word documents. Knowing what file extensions are is essential to your sanity, and your computer's. 

Here are a very few file extensions - .  If you have found a file with an extension you don't recognise, and want to know what it is, one way to find out (without buying a dictionary of file extensions, or searching on your computer's help menus) is to search on's file extension database on the web. Karl Resch also has a list of 1000 file extensions for that rainy November evening. 
.doc A document (file) created in Microsoft word
.txt A text only file - i.e. without any formatting such as indented paragraphs, heading hierarchies etc
.wbk Word backup file - a file that holds the last saved version of what you did, so that you can revert to it if you want
.jpg A "JPEG" (pronounced jay-peg) compressed image file, used widely on the internet since the compression results in a smaller file size
.gif A GIF file - another compressed file
.bmp Windows bitmap - an uncompressed file.  "Wallpaper" pictures on your desktop are usually in this format
.ram Real Audio Media file - 
.mp3 a compressed audio file
.mid A MIDI (musical instrument digital interface) file - technically not a sound file, since it does not "contain" sound, but instructions to musical instruments (such as a keyboard or the internal synthesizer in a computer) which will intern cause music to be played.
.wav An uncompressed sound file
.au Audio file
.exe Short for "executable file", program (application) files end in ".exe" (often known as "eksee files"). 
.dll Dynamic library link - a system file 
.sys System file
.ini Initialization file - contains information about how a program is set up - preferences and options, for example.
.zip, .z, .tar, .lzw A compressed file
.ico An icon file
.pdf (portable document format) A file that can be read by Acrobat Reader
.html, .htm (Hypertext Markup Language) A document formatted with special codes that can be read and interpreted by web browsers


Updated Sunday November 11, 2001 4:29 PM


© Jonathan Still 2001 You may quote from these pages, but if your selection includes a reference I have made to someone else's work, please make sure that the attribution is clear. By not doing so, you may implicate me in plagiarism.