Generalized Timing Formula is a standard by VESA which defines exact parameters of the component video signal for analog VGA display interface.
The parameters defined by the standard include horizontal blanking (retrace) and vertical blanking intervals, horizontal frequency and vertical frequency (collectively, pixel clock rate or video signal bandwidth), and horizontal/vertical sync polarity. These parameters are used by the XFree86 Modeline, for example. This standard is available from VESA for 350 USD. [1]
The standard was adopted in 1999, and was superseded by the Coordinated Video Timings specification in 2002.