CCITT Group 3

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File Format
Name CCITT Group 3
Extension(s) .g3
MIME Type(s) image/g3fax
PRONOM x-cmp/14

CCITT Group 3 is a lossless compressed data format for bi-level images. It is also known as T4, T.4, Fax3, Group 3, G3, or ITU-T Fax Group 3. It is one of the formats used by fax machines. It is usually embedded in another image file format such as TIFF, but can be stored in a file by itself.

It comes in two main varieties: so-called 1-dimensional and 2-dimensional (though both are used on 2-dimensional images). The 1-dimensional variety uses Modified Huffman (MH) compression, and the 2-dimensional variety uses Modified READ (MR) compression.


Related formats

Many of the formats listed in the Fax formats article use CCITT Group 3 compression.

In a TIFF file, compression codes 2, 3, and 32771 are varieties of CCITT Group 3 compression.

Other formats that can use CCITT Group 3 compression include PDF and SPIFF.

See also



Sample files


  • RFC 1494: Equivalences between 1988 X.400 and RFC-822 Message Bodies
  • RFC 2159: A MIME Body Part for FAX
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