The common metric and SI prefixes have been around for a while, but some were only recently added. In 1991, in order to apply standard units (SI units; see above) to a wide range of phenomena (especially in the scientific world), the Nineteenth General Conference on Weights and Measures lengthened the list to accommodate larger (and smaller) metric numbers—with the list now reaching from yotta- to yocto-. The following lists the American system (the name for large numbers) and the corresponding metric prefix and numerical equivalent (for comparison with prefixes and the power of ten, see “Math Basics”):

**American system** |
**metric prefix/symbol** |
**number** |

1 septillion |
yotta- / Y- |
10^{24} |

1 sextillion |
zetta- / Z- |
10^{21} |

1 quintillion |
exa- / E- |
10^{18} |

1 quadrillion |
peta- / P- |
10^{15} |

1 trillion |
tera- / T- |
10^{12} |

1 billion |
giga- / G- |
10^{9} |

1 million |
mega- / M- |
10^{6} |

1 thousand |
kilo- / k- |
10^{3} |

1 hundred |
hecto- / h- |
10^{2} |

1 ten |
deka- / da- |
10 |

1 tenth |
deci- / d- |
10^{-1} |

1 hundredth |
centi- / c- |
10^{-2} |

1 thousandth |
milli- / m- |
10^{-3} |

1 millionth |
micro-/ fx |
10^{-6} |

1 billionth |
nano- / n- |
10^{-9} |

1 trillionth |
pico- / p- |
10^{-12} |

1 quadrillionth |
femto- / f- |
10^{-15} |

1 quintillionth |
atto- / a- |
10^{-18} |

1 sextillionth |
zepto- / z- |
10^{-21} |

1 septillionth |
yocto- / y- |
10^{-24} |

It is interesting to note that “deca-” is the recommended spelling by the International System (SI), but the United States National Institute of Standards and Technology spells the prefix “deka-.” Thus, either one is considered by most references to be correct. There are also spelling variations between countries; for example, in Italy, hecto- is spelled etto- and kilo- is spelled chilo-. But the symbols remain standard through all languages. And as for other numbers in the metric system—such as 10^{5} or 10^{-5}—there are no set names or prefixes.