Normal (4×3) Screen Aspect Ratio
The following display formats are typically used with screens having the 4/3 width to height ratio of traditional television broadcasting. For each display mode the width and height in pixels, the total number of pixels, and the name from which the mnemonic was derived is given. The "H/V" ratio gives the ratio of the image width to height in pixels, as a fraction if exact, otherwise to two decimal places.

When adjusted to fill a screen with 4/3 width to height ratio, only those display modes with an H/V Ratio of precisely 4/3 will have "square pixels". If the pixel size ratio differs from the physical screen size ratio, the number of pixels per unit of length will differ in the vertical and horizontal dimensions, and circles drawn with a uniform radius in pixels will appear as ellipses on the screen. To avoid this distortion, graphics software must differentially scale images to compensate for the disparity between pixel and physical screen size ratios. Note that the ancient IBM CGA actually has a pixel ratio of 16/10, just like the hottest new wide-screen HDTV displays. If you blow up a CGA image to fill a large screen HDTV monitor, the pixels will be square, albeit about as big as your thumb!

The "Quad" and "Hex" designations are emerging as nomenclature for high-resolution displays. "Quad" refers to a mode with four times as many pixels (hence twice the vertical and horizontal size in pixels) as a previous mode, while "Hex" denotes a display with 16 times the pixels (four times the vertical and horizontal pixels).

Wide (16×9 or 16×10) Screen Aspect Ratio
High Definition Television (HDTV) specifies a screen width to height ratio of 16 to 9; digital broadcast HDTV uses a resolution of 1920×1080 pixels for high resolution mode, which conforms precisely to this ratio. (Actual broadcast images are encoded as 1920×1088 pixels because MPEG-2 encoding requires the number of vertical lines to be a multiple of 16.) The wider screen aspect ratio permits viewing widescreen movies without either wasting a large portion of the screen by a "letterbox" presentation or ghastly "pan and scan" adaptation to a 4 by 3 screen.

The following computer display modes are intended to be used with HDTV-style wide screen displays. Most of these modes have a pixel width to height ratio of 16 to 10, since that yields pixel array dimensions easier to cope with in computer hardware and software. (The WXGA mode, with the somewhat odd dimensions of 1366×768 pixels is, however, within one pixel of 16 by 9 ratio.) "Quad" and "Hex" denote higher resolution multiples of the base wide screen modes, as for the 4 by 3 modes above.

Nomenclature for wide screen display modes is far from settled. The mnemonics given above seem to be the most frequently encountered, but various manufacturers have their own schemes for denoting wide screen variants of basic display modes. The following table gives the forms I've seen, but you may encounter still others.