Writing numbers and units so nobody misreads them

Write “1mm” with the tiniest gap in the wrong place and it can turn into “1 m m”, two metres instead of one millimetre. Physics numbers span from 10⁻¹⁸ to 10¹⁸ and beyond, so sloppy writing is not a style problem — it changes the answer.

Scientific notation solves the size problem: every number becomes one non-zero digit before the decimal, times a power of ten. Prefixes solve the writing problem: instead of 1 × 10⁻³ m, physicists just write 1 mm.

On top of that, the book fixes eight small writing rules: capitalisation, spacing, powers on prefixed units. A value written by one student then means exactly the same thing to every examiner.