Video Graphics Array

The 18-bit palette comes from using 6 bits per primary color per pixel (6-bit, 6-bit, 6-bit) providing up to 262,144 total colors. 320x200 mode allowed 256 on screen. 640x480 mode allowed 16. Later "VGA" cards would support 640x480x256 (and 320x240x256) and use 8-bits per primary color per pixel (24-bit color, 16.7 million colors to select from). But this was not the original standard IBM created. These 'extended VGA' modes actual come from VESA. OR, it was actually an 8514/A or XGA card that provided "VGA" with more colors and different resolutions. 8514/A and XGA still use an 18-bit palette. 8514/A and XGA are essentially non-compatible standards so manufactures needed to bridge them in the hardware.

Note that composite color output was not part of the standard and results of using it could not properly claim the resolution and colors of the standard. Very, very few VGA hardwares had specific composite out. However, VGA monochrome standards allowed for 64 shades that would be accurately produced even on an analog composite monitor. All this IBM just wouldn't accept that business was done in color and was still providing grey options; this time for digital and analog. (They even used Charlie Chaplin as their mascot. It's all true boys and girls). Similarly, in 1998 IBM would decide that when everybody was using 16.7 million colors, children under 13 would want 16 colors. Soo... Progress?

VGA can use various segments of the system memory bus while leaving other segments unused. So just as EGA can by used simultaneously with CGA (also Plantronics) or MDA (also Hercules), VGA can in theory be used with another type of video card to provide dual monitor.