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Q&A When to not using SI symbols for units?

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: In one line both mm and millim...

0 answers  ·  posted 8y ago by Ooker‭  ·  last activity 5y ago by System‭

#4: Attribution notice removed by user avatar System‭ · 2020-01-26T07:20:01Z (almost 5 years ago)
Source: https://writers.stackexchange.com/q/25305
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added by user avatar System‭ · 2019-12-08T05:45:24Z (almost 5 years ago)
Source: https://writers.stackexchange.com/q/25305
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision by (deleted user) · 2019-12-08T05:45:24Z (almost 5 years ago)
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:

[![enter image description here](https://i.stack.imgur.com/jJ2Lg.png)](https://i.stack.imgur.com/jJ2Lg.png)

In one line both _mm_ and _millimeter_ are even used together: ![enter image description here](https://i.imgur.com/j0nFZO4.png)

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?

#1: Imported from external source by user avatar System‭ · 2016-11-21T16:50:03Z (about 8 years ago)
Original score: 1