整理了一下。忘记了也有地方可以查

**Math spacing:**

\quad | width of M |

\! | -3/18 quad |

\, | 3/18 quad |

\: | 4/18 quad |

\; | 5/18 quad |

\ | 6/18 quad |

\qquad | 2 quad |

**Lengths defined relative to characters:**

em | width of M | |

ex | width of x | |

mu | 1 mu = 1/18 em |

**Physical lengths (commonly used):**

mm | millimeter | |

cm | centimeter | |

in | inch | 1 in = 25.4 mm |

pt | point | 1 pt = 1/72.27 in |

**Physical lengths (not so commonly used):**

bp | big point | 1 bp = 1/72 in |

pc | pica | 1 pc = 12 pt |

sp | scaled point | 1 sp = 1/65536 pt |

dd | didot point | 1 dd = 1238/1157 pt |

cc | cicero | 1 cc = 12 dd |

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em and \quad are the same, but they are only approximately the width of M. They are rather defined to be the size of the font, that is 1 em in a 12pt font is exactly 12pt wide.