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For ease of reference, the surfaces of planetary bodies can be subdivided into a series of areas distributed as regularly as possible across the surface of a planet or moon. The resulting segments are known as quadrangles because they usually have four sides. Away from the equator, the northern and southern sides of a quadrangle take on the shape of a circular section, so that the features within it can be mapped more accurately. At the highest latitudes, two circular 'quadrangles' are typically centred on the poles themselves.
The number of quadrangles used to map a surface will depend on the scale required. At the lowest resolutions, as few as eight quadrangles can be used, or even just three in extreme cases, with each covering a significant proportion of the surface. More detailed systems will divide a surface into (to take some common examples) 30, 62, or 144 segments, each of comparable area.
Each quadrangle on a system is given a code to identify the body in question and a quadrangle number. So, for example, quadrangle 'V-1' covers the north pole of Venus, while MC-09 contains (most of) Olympus Mons on Mars. Quadrangles are also often given names from prominent features within then, so V-1 is also known as the Snegurochka Planitia quadrangle from Venus' North Polar Plains, while MC-09 is the Tharsis quadrangle (from the name of the Martian region where Olympus Mons stands).
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