HTM or HTML Extension?

[Edu India]

HTM is used only as an alternate extension to .HTML. This happens for some reasons, for examples, in some operating systems, like the Disk Operating System and Window 3.X; they do not allow the use of four-letter extensions.
HTML for some operating systems and servers that do not accept four-letter extensions.
Although today, operating systems have developed, and can now support long file names and four-letter file extensions.
HTM and HTML refer to file extensions of HTML files. They are files of a plain text type.