The ITU-T Group 1 to 4 specify compression of black and white documents and the operation of facsimile equipment.

Group 3 (also known as G3 or T.4) is presently the most important standard in the world of fax and document storage applications. G3 compression features modified Huffman encoding. The ITU-T Group 4 (also known as G4 or T.6) is an improvement of ITU-T G3, dedicated to digital telephone lines, in particular ISDN.