A TIFF file begins with an 8-byte image file header, containing the following information:
Bytes 0-1: The byte order used within the file. Legal values are:
"II" (4949.H)
"MM" (4D4D.H)
In the "II" format, byte order is always from least significant byte to most significant byte, for both 16-bit and 32-bit integers. This is called little-endian byte order.
In the "MM" format, byte order is always from most significant to least significant, for both 16-bit and 32-bit integers. This is called big-endian byte order.
In both formats, character strings are stored in sequential bytes, and are null terminated.
All baseline TIFF readers must support both byte orders. A TIFF writer may use the one that is most convenient.
Bytes 2-3 An arbitrary but carefully chosen number (42) that further identifies the file as a TIFF file.
Bytes 4-7 The offset (in bytes) of the first Image File Directory. The directory may be at any location in the file after the header but must begin on a word boundary. In particular, an Image File Directory may follow the image data it describes. Readers must follow the pointers, wherever they may lead.
The term byte offset is always used in this document to refer to a location with respect to the beginning of the TIFF file. The first byte of the file has an offset of 0.