Yeah, the ghost of Jean-Charles de Borda is rather wordy, isn't he? It boils down to: 1) Fundamental units are the base units by which all other types of meas

Yeah, the ghost of Jean-Charles de Borda is rather wordy, isn't he? It boils down to: 1) Fundamental units are the base units by which all other types of measurement units are defined. 2) The kilogram is the last fundamental unit that is defined by an arbitrary physical artifact. This is a problem because every time it's taken out to calibrate other measurements, the measurement process wears it out a little, making its mass change. 3) Other units are defined now by simple arbitrarily chosen values and the interrelationship between those values as defined by the formulae of physics. For example, a second is defined as "The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom." 4) The particular physics formula chosen to use as part of the definition matters greatly because the precision of the definition depends on the least precise measurement of the other variables of the formula being used. 5) After long debate, the standard definition of the kilogram is about to be changed to a definition based on an arbitrarily chosen value of Planck's constant, which is good because it's now reproducible everywhere instead of being dependent on a physical object that's slowly being worn out.