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In the book What If?, the author usually write the unit as kilometer, atmosphere, megawatt instead of km, atm, MW. However he does use the symbols in the drawings:

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In one line both mm and millimeter are even used together: enter image description here

This makes me confused because I can't find a reason to not using the SI symbols. I guess the reason is similar to writing numbers in words when doing so doesn't look silly for better reading flow feeling. But in the above example, I can't explain why the first number should use mm while the second one should use millimeter.

Is there a general rule for writing out the unit instead of its SI symbol?

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    Yes, be consistent. Which this writer wasn't. It's sloppy. – Lauren-Reinstate-Monica-Ipsum Nov 21 '16 at 22:44
  • but surely the book had been reviewed by some editors. They should acknowledge of this, right? – Ooker Nov 25 '16 at 12:40
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    Well first, it's XKCD, so you could as Randall Munroe directly. It's not intended to be for engineers or scientists, it's written for average Joes. And to me it reads like a clarification, that a bullet is really really fast. He could have written mm/ms, but that wouldn't look fast. It doesn't need to be consistent, it needs to sell. – Alexander von Wernherr Dec 5 '16 at 6:16
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    As @AlexandervonWernherr you can always ask the author. Worst thing that could happen is ending up in a cartoon ;) – Helmar Dec 5 '16 at 13:19
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    @Helmar I sent an email a while ago, but her haven't answered it yet :( – Ooker Dec 5 '16 at 15:36
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The most important thing (at least, in almost all circumstances) is consistency. Most of the time, as long as you're consistent, you'll be forgiven for whatever choice you make.

If you're writing in a particular context, and other people writing in that context write the units in a certain way, do that. If you know of some specific reason why a certain choice is wiser or more appropriate than another, do that. Otherwise, just take your pick and be consistent.

"What if?" is essentially a comedy book (or at least, due to its association with a well known humorous web-comic, comes with some expectation of comedic content), so it's very possible the writer is being deliberately inconsistent for comedic effect.

Also possible is that the writer and editor both simply missed it. It happens.

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If you want to know the standard way {1}, see: http://physics.nist.gov/cuu/pdf/sp811.pdf gives the rules, including capitalization and abbreviations. {1} NIST was formerly the National Bureau of Standards

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