What is the difference between the HTM and HTML extensions?

HTM is just a shorter way of writing HTML, but both are the same type of extention file name. But html is best that you use because it is just a lazy way of writing the extension. And it can be that much more work to add an L at the end can it?

At the saving time of html file, we can use either the .html or the .htm extension. If we are using .htm extention in our html page, it might be a bad habit inherited from the past when some of the commonly used software only allowed three letter extensions.

It will be better to use .HTML extension for the newer software.