Sometimes you'll see an HTML file as ".html," but it is more frequent to see the 3 letter file extension of ".htm" used.

File extensions, such as htm, are almost always three letters and usually lower case. Other common extensions include the following the file called example:

• example.txt: for text files
• example.doc: for Microsoft Word files
• example.jpg: for JPEGs, a type of image file
• example.dll: for a dynamic link library used by Microsoft OSs
• example.exe: for executable files
• example.htm for an HTML file

The extensions may be derived from the file's function, creator, company or some other associated word or action.