What you cannot measure, you cannot make
What happens when rulers disagree? It sounds trivial when you are told; it is different when you measure.
The wooden stick at the top is measured with the two rulers below. One division on my ruler is a different size from one on my neighbour's. Use the slider to change the size of a division on mine.
One division of my ruler medium
I call out the number I measured, and my neighbour cuts that many divisions by their own ruler and brings it over.
This really happened. One team sent its figures in its own country's units, and the other team read those numbers as international units. Both had calculated correctly. Open the slider by as much as the two rulers disagreed.
How far the two teams' rulers disagree the same
The far right is how far apart they really were. The unit of force one side used was four and a half times the other's.
Everything in the world that can be measured comes from these seven. Tap them one by one.
Six of the seven moved over to values of nature long ago; the last one still tied to an object was the kilogram. That one moved over in 2019. There is now no unit on Earth defined by an object.
The place that keeps these seven for Korea is the Korea Research Institute of Standards and Science, in Yuseong-gu, Daejeon. A copy modelled on the prototype in the French vault is kept here too. It has given up its place as the standard, but it is still carefully looked after. The atomic clocks that make and keep Korea Standard Time, the time you see on your phone, are also here. Train timetables and the records of financial transactions all run to these clocks.
It reads far more easily after you have tried it.
To agree on units is to make it possible to trust one another. Only when someone else can use the value I measured can things be made in separate places, and other people's experiments be repeated. Measuring may look trivial, but everything else rests on it.