CCITT Group 4 compression, usually abbreviated as G4, is a method of image compression used in Group 4 fax machines, and document scanners defined in the ITU-TT.6 fax standard. It is only used for bitonal (black and white) images. G4 compression offers a small improvement over G3-2D by removing the end of line (EOL) codes. G3 and G4 compression both treat an image as a series of horizontal black strips on a white page. Better compression is achieved when there are fewer unique black dots/lines on the page. Both G3-2D and G4 add a two dimensional feature to achieve greater compression by taking advantage of vertical symmetry. A worst-case image would be an alternating pattern of single pixel black and white dots offset by one pixel on even/odd lines. G4 compression would actually increase the file size on this type of image. G4 compresses the average file at an approximate rate of 20:1. An example is an 8.5"x11" page is scanned at 200DPI. It is 456.54KB (uncompressed) in size. Once G4 compresses it, it is reduced to 22.82KB (456.54KB\20 = 22.82KB). It is available in many proprietary image file formats as well as standardized formats such as TIFF, CALS, and the PDF document format.