Remember the SI units we learn in physics lessons way back in our high school years?
The length that set the standards for the meter we use was actually a platinum bar now still kept in France.
But since October 20th 1960 (exactly 29 years before I was born) The length for the standard metre has been redefined in the 11th General Conference on Weights and Measures as:
"1,650,763.73 wavelengths in vacuum of the radiation (light) corresponding to the transition between the 2p10 and 5d5 quantum levels of the Krypton-86 atom."
Using the multiple distance of wavelengths of light to measure distance, now that's precision!