The Grand K is a cylinder made of a platinum-iridium alloy that is stored in three locked bell jars in environmentally monitored safe, which itself is kept in a vault in the International Bureau of Weights and Measures Pavillon de Breteuil, Saint-Cloud in the western suburbs of Paris.
It was manufactured in 1879. A decade later, it became the official global standard weight of one kilogramme.
The weight has been removed from storage only four times since it was created so that official replicas can be measured against it. The problem is, despite all efforts to keep the weight constant, tests prove that it is not the same mass as it was in 1889, and it is different to its replicas.
The differences are not enough to worry consumers in their everyday lives, but they can affect scientific calculations.
The Grand K is the last of the official units of measurement to rely on a physical standard, rather than a system of calculation that can be replicated anywhere.
Scientists now believe they can work out the mass using a calculation taking in Einstein's theory of relativity and Planck's constant - a vital component in the field of quantum mechanics.
The maths has been understood for some time, but experts now believe they have calculated Planck's constant accurately enough to improve on the weight standard set in 1889.
If these calculations, which were submitted by multiple teams earlier this month, are approved, the Grand K could be relegated to history by 2020, where it will join the metre bar - also made of the platinum-iridium alloy - which from 1791 until 1960 was the physical official definition of the length of a metre.
That measure is now defined as the distance travelled by a particle of light in 1/299,792,458 of a second.