A byte order mark (BOM) is a sequence of bytes used to indicate the Unicode encoding style of a text file. The encoding dictates how text is serialized into a sequence of bytes. If the least significant byte is placed in the initial position, this is referred to as "little-endian," whereas if the most significant byte is placed in the initial position, the method is known as "big-endian."
In addition to indicating the byte order, a BOM can also be used as a file signature to identify the encoding of a text file. The UTF-8 file signature (commonly also referred to as a "BOM") identifies the encoding format rather than the byte order of the document. UTF-8 is a linear sequence of bytes (not a sequence of 2-byte or 4-byte units where the byte order is important as in UTF-16 and UTF-32). The following table shows the byte-order marks for various encodings.
|Byte Order Mark (BOM)||Encoding Form|
|EF BB BF||UTF-8|
|FE FF||UTF-16, big-endian|
|FF FE||UTF-16, little-endian|
|00 00 FE FF||UTF-32, big-endian|
|FF FE 00 00||UTF-32, little-endian|
BOM use is optional. If used, it must be at the very beginning of the text. The BOM gives the producer of the text a way to describe the encoding such as UTF-8 or UTF-16, and in the case of UTF-16 and UTF-32, its endianness. The BOM is important for text interchange, when files move between systems that use different byte orders or different encodings, rather than in normal text handling in a closed environment.
As UTF-8 has become the most common text encoding,
EFBBBF (shown here as three hexadecimal values) is the most commonly occurring BOM form, also known as the UTF-8 signature. HTML5 browsers are required to recognize the UTF-8 BOM and use it to detect the encoding of the page. Software may alternatively recognize UTF-8 encoding by looking for bytes with the high order bit set (values
0xFF) followed by bytes that define valid UTF-8 sequences.
Most modern software applications recognize a BOM and may insert it when saving a text file with UTF encoding. The presence of the UTF-8 BOM may cause problems with some software, especially legacy software not designed to handle UTF-8, in which case it may appear as the characters "ï»¿".