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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: In one line both mm and millim...
Question
technical-writing
#4: Attribution notice removed
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
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
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?