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Pixels and Resolution
The image that is displayed on the screen is composed of thousands (or millions) of small dots; these are called pixels; the word is a contraction of the phrase "picture element". A pixel represents the smallest piece of the screen that can be controlled individually. Each one can be set to a different color and intensity (brightness).
The number of pixels that can be displayed on the screen is referred to as the resolution of the image; this is normally displayed as a pair of numbers, such as 640x480. The first is the number of pixels that can be displayed horizontally on the screen, and the second how many can be displayed vertically. The higher the resolution, the more pixels that can be displayed and therefore the more that can be shown on the monitor at once, however, pixels are smaller at high resolution and detail can be hard to make out on smaller screens. Resolutions generally fall into predefined standard sets; only a few different resolutions are used by most PCs.
The aspect ratio of the image is the ratio of the number of X pixels to the number of Y pixels. The standard aspect ratio for PCs is 4:3, but some resolutions use a ratio of 5:4. Monitors are calibrated to this standard so that you can draw a circle and have it appear to be a circle and not an ellipse. Displaying an image that uses an aspect ratio of 5:4 will cause the image to appear somewhat distorted. The only mainstream resolution that currently uses 5:4 is the high-resolution 1280x1024.
There is some confusion regarding the use of the term "resolution", since it can technically mean different things. First, the resolution of the image you see is a function of what the video card outputs and what the monitor is capable of displaying; to see a high resolution image such as 1280x1024 requires both a video card capable of producing an image this large and a monitor capable of displaying it. Second, since each pixel is displayed on the monitor as a set of three individual dots (red, green and blue), some people use the term "resolution" to refer to the resolution of the monitor, and the term "pixel addressability" to refer to the number of discrete elements the video card produces. In practical terms most people use resolution to refer to the video image, as I do on this site.
The table below lists the most common resolutions used on PCs and the number of pixels each uses: