Ordinary rulers stop at the millimetre. To measure germs we need smaller units. Each one is 1,000 times smaller than the one before:

• 1 millimetre (mm) = 10⁻³ m, one thousandth of a metre
• 1 micrometre (µm) = 10⁻⁶ m = 0.001 mm, one millionth of a metre
• 1 nanometre (nm) = 10⁻⁹ m = 0.001 µm, one billionth of a metre