When writing a file and the following conditions are met, a BOM (Byte Order Mark) is prepended to the file:

• this option is on
• the 'binary' option is off
• 'fileencoding' is "utf-8", "ucs-2", "ucs-4" or one of the little/big endian variants. Some applications use the BOM to recognize the encoding of the file. Often used for UCS-2 files on MS-Windows. For other applications it causes trouble, for example: "cat file1 file2" makes the BOM of file2 appear halfway the resulting file. Gcc doesn't accept a BOM. When Vim reads a file and 'fileencodings' starts with "ucs-bom", a check for the presence of the BOM is done and 'bomb' set accordingly. Unless 'binary' is set, it is removed from the first line, so that you don't see it when editing. When you don't change the options, the BOM will be restored when writing the file.

(default off)