Mode X is an alternative graphics display mode of the IBM VGA graphics hardware that was popularized by Michael Abrash. It was first published in July 1991 in Dr. Dobb's Journal, and republished in chapters 47-49 of Abrash's Graphics Programming Black Book (now freely available online). The term "Mode X" was coined by Abrash.
The primary advantage of Mode X is that it has square pixels: a resolution of 320x240 instead of the standard VGA Mode 13h which is 320x200. Additionally, Abrash enabled the VGA's planar memory mode (also called "unchained mode"). Even though planar memory mode is a documented part of the VGA standard and was used in earlier commercial games, it was first widely publicized in the Mode X articles, leading many programmers to consider Mode X and planar memory synonymous. It is possible to enable planar memory in standard 320x200 mode, which became known as "Mode Y" in the Usenet rec.games.programmer group.
Planar memory arrangement splits the pixels horizontally into groups of four. For any given byte in the PC video memory aperture, you can access four pixels on screen, by selecting the plane(s) you require. This is more complicated for the programmer, but the advantages gained by this arrangement were considered very worthwhile by many, and Mode X found considerable use in demos and 2D games.
"Mode Q" ("Q" being short for "cube") is sometimes used to refer to Chain-4 256x256 256 colour mode. This mode's advantage is the simplistic and fast method for addressing pixels in memory based on the X and Y coordinates. The Y coordinate can simply be put in the high byte of the address, and the X coordinate in the low byte, forming the address of the pixel without a multiply, shift or lookup being needed like other video modes.