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The first prefix is numberderived; second, third, and fourth are based on mythology. Fifth and sixth are supposed to be just that: fifth and sixth. But, with the seventh, another fork has been taken. The General Conference of Weights and Measures (CGMP, from the French; they have been headquartered, since 1874, in Sevres on the outskirts of Paris) has now decided to name the prefixes, starting with the seventh, with the letters of the Latin alphabet, but starting from the end. Now, that makes it all clear! Remember, both according to CGMP and SI, the prefixes refer to powers of 10. Mega is 10**6, exactly 1,000,000, kilo is exactly 1000, not 1024.
Sources: Various books, the Internet, and various encyclopedias.
Kilder: Forskellige bøger, internettet og forskellige leksikoner.
