= System of units
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The key thing in a good system of units is to define units in a way that depends only on physical properties of nature.
Ideally (or basically necessarily?) the starting point generally has to be discrete phenomena, e.g.
* number of times some light oscillates per second
* number of steps in a <quantum Hall effect> or <Josephson junction>
What we don't want is to have macroscopics measurement artifacts, (or even worse, the size of body parts! Inset <dick> joke) as you can always make a bar slightly more or less wide. And even metals evaporate over time! Though the mad people of the <Avogadro project> still attempted otherwise well into the 2010s!
Standards of measure that don't depend on artifacts are known as <intrinsic standards>.