Why do some file formats have multiple extensions?

Actually I just learned this a few weeks ago: .htm is the newer version of .html

I agree with boba0420

It is just throughout the passage of time.

Remember when 2003 word documents were in the .doc format? It can still be used in 2007 and 2010 office, but just an older way of doing it. The current one now being .docx

Same thing with Applications. There used to be two extensions: .com and .exe. Now we jsut have .exe

Or even (I dare say it) batch files. I believe in win95, they added the .cmd version along with the original .bat extension. Don't know if there was much of a difference, but who cares?

from http://www.sightspecific.com/~mosh/WWW_FAQ/ext.html

Technically speaking there are few to no important differences. An obvious difference is the addition of the letter "L" in the html extension. The technical difference that the additional letter will make to the operating system is better left to a different discussion but in the context of a web author, the additional "L" will make no difference.

The technical difference that the additional letter will make to an http server (a "web server") is minimal. Usually, a server will use a file's extension to figure out what MIME type to send back to the requesting client. Most servers are configured by default to send back the text/html type when the requested file ends in an htm or html extension. This can be changed by the server's administrator(s) in such a way that one of the above extensions returns a different MIME type then the other, however, this is not a very common practice.

It is a common misconception that a file ending in an htm extension had to have been created on a DOS/Windows 3.x platform. This is because those operating environments limit filenames to a 3 letter extension. However, it is very simple to create a file with a 3 letter extension on most other platforms, as well. Even those that allow longer file extensions.