If you want to open a MAC OS X file in Windows, you may need to add the extension to a MAC OS X file name so Windows can recognize it. For example, the doc extension tells your computer that the file is a Microsoft Word file.
For example, in letter.docx the filename is letter and the extension is docx. Extensions are important because they tell your computer what icon to use for the file, and what application can open the file. The extension is a three- or four-letter abbreviation that signifies the file type. Windows file names have two parts the file's name, then a period followed by the extension (suffix).
