Here's the breakdown of the metric prefixes related to meters, including what comes before nanometer:

* Kilometer (km): 1,000 meters

* Hectometer (hm): 100 meters

* Dekameter (dam): 10 meters

* Meter (m): 1 meter (base unit)

* Decimeter (dm): 0.1 meters

* Centimeter (cm): 0.01 meters

* Millimeter (mm): 0.001 meters

* Micrometer (µm): 0.000001 meters

* Nanometer (nm): 0.000000001 meters

So, the unit that comes before nanometer is micrometer (µm).