Tại sao không thể đặt tên cho thư mục ‘._.’ Trong Windows 7?

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Windows normally requires files to have either no extension or an extension that is at least one character long. It does not like zero length extensions (i.e. file names that end with a period). Folders can have extensions too, therefore, Windows does not let their names end with a period.

Source: From the article linked to by DavidPostill:

Use a period to separate the base file name from the extension in the name of a directory or file.

If you try to end a file or directory name with a period, Windows just assumes that you did not want an extension and thus removes it (even if you create it with the md command in a command prompt window).

Danger Zone

If you really want a folder name to end with period, you will need to use the magic raw name override sequence of ?. In a command prompt window, md \?C:path ocontainer._. will indeed create a folder named ._., but lots of programs will have problems with it, even Windows Explorer:

Such a directory can only be removed with the rd command followed by its ? name, or renamed with its short (8.3, dir /x) name.