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Generalized Timing Formula is a standard by VESA which defines exact parameters of the component video signal for analogue VGA display interface.
The video parameters defined by the standard include horizontal blanking and vertical blanking intervals, horizontal frequency and vertical frequency , and horizontal/vertical sync polarity. Unlike predefined discrete modes , any mode in a range can be produced using a formula by GTF.
A GTF-compliant display is expected to calculate the blanking intervals from the signal frequencies, producing a properly centered image. At the same time, a compliant graphics card is expected to use the calculation to produce a signal that will work on the display — either a GTF default formula for then-ordinary CRT displays or via a custom formula provided via EDID signaling.
These parameters are used by the XFree86 Modeline, for example.