Modeline (X11)
A line in the X server configuration giving the pixel clock and sync pulses for a video mode, from the era when a monitor could not say for itself what it could survive.
Before monitors had EDID, VESA's data channel from 1994, the X server knew nothing about what sat at the other end of the cable. So the user wrote Modeline "1024x768" 65.00 1024 1048 1184 1344 768 771 777 806 -hsync -vsync: the pixel clock in megahertz, then four numbers for the horizontal line (visible pixels, start and end of the sync pulse, total line length) and four for the vertical, followed by sync polarity. Pixel clock divided by line length gives the horizontal scan rate, and that divided by frame height gives the refresh rate: 48.4 kHz and 60 Hz in the example.
Values beyond what a monitor tolerated could damage older fixed-frequency CRTs, which is why the XFree86 documentation warned about it in capitals. Eric S. Raymond's Video Timings HOWTO taught a generation to work the numbers out by hand; xvidtune let you nudge the picture interactively, and gtf and cvt compute a line from VESA's formulas. Today Modelines are mostly needed for odd displays and resolutions the EDID data leaves out, and they are added at runtime with xrandr --newmode rather than in xorg.conf.