Read "The SI Base Units" and "The SI Decimal Prefixes" sections. Note that the base unit for mass (kg) is the only "base" unit with a prefix. This arises from the fact that the "gram" is so small (i.e., a nickel weighs approximately 5 grams) that "real world" objects cannot feasibly be measured in grams. Can you imagine how many grams a table would weigh? Also, with the exception of seconds, the SI system (System International) is a base-10 system, allowing easier conversion between units.