To save your model to a disk file, use the File|Save command, or press the Save button () in the toolbar. Unless you specify otherwise, LINGO will automatically append a .LG4 extension to your file name on Windows.  The .LG4 file format is a binary RTF file format.  On Mac and Linux, LINGO  automatically saves model in the .LNG file format, which is a plain text file format, generally readable by any other programs that can read text files.